The behavior of btrfs_delalloc_reserve_metadata depends on whether
the inode we are allocating for is the freespace inode or not. As it
stands if we are the free node we set 'flush' and 'delalloc_lock'
variable to certain values. Subsequently we check the values of those
vars and act accordingly. Instead, simplify things by having 1 if
which checks whether we are the freespace inode or not and do any
specific operation in either branches of that if. This makes the code
a bit easier to understand, as an added bonus it also shrinks the
compiled size:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-17 (-17)
Function                                     old     new   delta
btrfs_delalloc_reserve_metadata             1876    1859     -17
Total: Before=85966, After=85949, chg -0.02%

No functional changes.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Edmund Nadolski <enadolski@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>