btrfs: scrub: fix uninitialized return value in recover_scrub_rbio

Commit 75b47033 ("btrfs: raid56: migrate recovery and scrub recovery
path to use error_bitmap") introduced an uninitialized return variable.

This can be caught by gcc 12.1 by -Wmaybe-uninitialized:

  CC [M]  fs/btrfs/raid56.o
fs/btrfs/raid56.c: In function ‘scrub_rbio’:
fs/btrfs/raid56.c:2801:15: warning: ‘ret’ may be used uninitialized [-Wmaybe-uninitialized]
 2801 |         ret = recover_scrub_rbio(rbio);
      |               ^~~~~~~~~~~~~~~~~~~~~~~~
fs/btrfs/raid56.c:2649:13: note: ‘ret’ was declared here
 2649 |         int ret;

The warning is disabled by default so we haven't caught that.

Due to the bug the raid56 scrub fstests have been failing since the
patch was merged, so initialize that.

Fixes: 75b47033 ("btrfs: raid56: migrate recovery and scrub recovery path to use error_bitmap")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>