btrfs: unexport repair_io_failure()

Function repair_io_failure() is no longer used out of extent_io.c since
commit 8b9b6f25

 ("btrfs: scrub: cleanup the remaining nodatasum
fixup code"), which removes the last external caller.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>