btrfs: add dummy callback for readpage_io_failed and drop checks

Make extent_io_ops::readpage_io_failed_hook callback mandatory and
define a dummy function for btrfs_extent_io_ops. As the failed IO
callback is not performance critical, the branch vs extra trade off does
not hurt.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -4658,7 +4658,7 @@ static const struct extent_io_ops btree_extent_io_ops = {
 	.readpage_end_io_hook = btree_readpage_end_io_hook,
 	/* note we're sharing with inode.c for the merge bio hook */
 	.merge_bio_hook = btrfs_merge_bio_hook,
+	.readpage_io_failed_hook = btree_io_failed_hook,
 
 	/* optional callbacks */
-	.readpage_io_failed_hook = btree_io_failed_hook,
 };