[BUG]
Although dev replace ioctl has a way to specify the mode on whether we
should read from the source device, it's not properly followed.

 # mkfs.btrfs -f -d raid1 -m raid1 $dev1 $dev2
 # mount $dev1 $mnt
 # xfs_io -f -c "pwrite 0 32M" $mnt/file
 # sync
 # btrfs replace start -r -f 1 $dev3 $mnt

And one extra trace is added to scrub_submit(), showing the detail about
the bio:

  btrfs-11569 [005] ...  37.0270: scrub_submit.part.0: devid=1 logical=22036480 phy=22036480 len=16384
  btrfs-11569 [005] ...  37.0273: scrub_submit.part.0: devid=1 logical=30457856 phy=30457856 len=32768
  btrfs-11569 [005] ...  37.0274: scrub_submit.part.0: devid=1 logical=30507008 phy=30507008 len=49152
  btrfs-11569 [005] ...  37.0274: scrub_submit.part.0: devid=1 logical=30605312 phy=30605312 len=32768
  btrfs-11569 [005] ...  37.0275: scrub_submit.part.0: devid=1 logical=30703616 phy=30703616 len=65536
  btrfs-11569 [005] ...  37.0281: scrub_submit.part.0: devid=1 logical=298844160 phy=298844160 len=131072
  ...
  btrfs-11569 [005] ...  37.0762: scrub_submit.part.0: devid=1 logical=322961408 phy=322961408 len=131072
  btrfs-11569 [005] ...  37.0762: scrub_submit.part.0: devid=1 logical=323092480 phy=323092480 len=131072

One can see that all the reads are submitted to devid 1, even if we have
specified "-r" option to avoid reading from the source device.

[CAUSE]
The dev-replace read mode is only set but not followed by scrub code at
all.  In fact, only common read path is properly following the read
mode, but scrub itself has its own read path, thus not following the
mode.

[FIX]
Here we enhance scrub_find_good_copy() to also follow the read mode.

The idea is pretty simple, in the first loop, we avoid the following
devices:

- Missing devices
  This is the existing condition

- The source device if the replace wants to avoid it.

And if above loop found no candidate (e.g. replace a single device),
then we discard the 2nd condition, and try again.

Since we're here, also enhance the function scrub_find_good_copy() by:

- Remove the forward declaration

- Makes it return int
  To indicates errors, e.g. no good mirror found.

- Add extra error messages

Now with the same trace, "btrfs replace start -r" works as expected:

  btrfs-1213 [000] ...  991.9059: scrub_submit.part.0: devid=2 logical=22036480 phy=1064960 len=16384
  btrfs-1213 [000] ...  991.9062: scrub_submit.part.0: devid=2 logical=30457856 phy=9486336 len=32768
  btrfs-1213 [000] ...  991.9063: scrub_submit.part.0: devid=2 logical=30507008 phy=9535488 len=49152
  btrfs-1213 [000] ...  991.9064: scrub_submit.part.0: devid=2 logical=30605312 phy=9633792 len=32768
  btrfs-1213 [000] ...  991.9065: scrub_submit.part.0: devid=2 logical=30703616 phy=9732096 len=65536
  btrfs-1213 [000] ...  991.9073: scrub_submit.part.0: devid=2 logical=298844160 phy=277872640 len=131072
  btrfs-1213 [000] ...  991.9075: scrub_submit.part.0: devid=2 logical=298975232 phy=278003712 len=131072
  btrfs-1213 [000] ...  991.9078: scrub_submit.part.0: devid=2 logical=299106304 phy=278134784 len=131072
  ...
  btrfs-1213 [000] ...  991.9474: scrub_submit.part.0: devid=2 logical=318504960 phy=297533440 len=131072
  btrfs-1213 [000] ...  991.9476: scrub_submit.part.0: devid=2 logical=318636032 phy=297664512 len=131072
  btrfs-1213 [000] ...  991.9479: scrub_submit.part.0: devid=2 logical=318767104 phy=297795584 len=131072
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>