btrfs: make btrfs_check_repairable() static

Since its introduction in commit 2fe6303e ("Btrfs: split
bio_readpage_error into several functions"), btrfs_check_repairable()
has only been used from extent_io.c where it is defined.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>