When inserting a new extent state record into an io tree that happens to
be mergeable, we currently do the following:

1) Insert the extent state record in the io tree's rbtree. This requires
   going down the tree to find where to insert it, and during the
   insertion we often need to balance the rbtree;

2) We then check if the previous node is mergeable, so we call rb_prev()
   to find it, which requires some looping to find the previous node;

3) If the previous node is mergeable, we adjust our node to include the
   range of the previous node and then delete the previous node from the
   rbtree, which again may need to balance the rbtree;

4) Then we check if the next node is mergeable with the node we inserted,
   so we call rb_next(), which requires some looping too. If the next node
   is indeed mergeable, we expand the range of our node to include the
   next node's range and then delete the next node from the rbtree, which
   again may need to balance the tree.

So these are quite of lot of iterations and looping over the rbtree, and
some of the operations may need to rebalance the rb tree. This can be made
a bit more efficient by:

1) When iterating the rbtree, once we find a node that is mergeable with
   the node we want to insert, we can just adjust that node's range with
   the range of the node to insert - this avoids continuing iterating
   over the tree and deleting a node from the rbtree;

2) If we expand the range of a mergeable node, then we find the next or
   the previous node, depending on other we merged a range to the right or
   to the left of the node we are currently at during the iteration. This
   merging is as before, we find the next or previous node with rb_next()
   or rb_prev() and if that other node is mergeable with the current one,
   we adjust the range of the current node and remove the other node from
   the rbtree;

3) Whenever we need to insert the new extent state record it's because
   we don't have any extent state record in the rbtree which can be
   merged, so we can remove the call to merge_state() after the insertion,
   saving rb_next() and rb_prev() calls, which require some looping.

So update the insertion function insert_state() to have this behaviour.

Running dbench for 120 seconds and capturing the execution times of
set_extent_bit() at pin_down_extent(), resulted in the following data
(time values are in nanoseconds):

Before this change:

  Count: 2278299
  Range:  0.000 - 4003728.000; Mean: 713.436; Median: 612.000; Stddev: 3606.952
  Percentiles:  90th: 1187.000; 95th: 1350.000; 99th: 1724.000
       0.000 -       7.534:       5 |
       7.534 -      35.418:      36 |
      35.418 -     154.403:     273 |
     154.403 -     662.138: 1244016 #####################################################
     662.138 -    2828.745: 1031335 ############################################
    2828.745 -   12074.102:    1395 |
   12074.102 -   51525.930:     806 |
   51525.930 -  219874.955:     162 |
  219874.955 -  938254.688:      22 |
  938254.688 - 4003728.000:       3 |

After this change:

  Count: 2275862
  Range:  0.000 - 1605175.000; Mean: 678.903; Median: 590.000; Stddev: 2149.785
  Percentiles:  90th: 1105.000; 95th: 1245.000; 99th: 1590.000
       0.000 -      10.219:      10 |
      10.219 -      40.957:      36 |
      40.957 -     155.907:     262 |
     155.907 -     585.789: 1127214 ####################################################
     585.789 -    2193.431: 1145134 #####################################################
    2193.431 -    8205.578:    1648 |
    8205.578 -   30689.378:    1039 |
   30689.378 -  114772.699:     362 |
  114772.699 -  429221.537:      52 |
  429221.537 - 1605175.000:      10 |

Maximum duration (range), average duration, percentiles and standard
deviation are all better.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>