Btrfs: use btrfs_op instead of bio_op in __btrfs_map_block

This seems to be a leftover of commit cf8cddd38b ("btrfs: don't
abuse REQ_OP_* flags for btrfs_map_block").

It should use btrfs_op() helper to provide one of 'enum btrfs_map_op'
types.

Fixes: cf8cddd38b ("btrfs: don't abuse REQ_OP_* flags for btrfs_map_block")
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6166,7 +6166,7 @@ int btrfs_map_bio(struct btrfs_fs_info *fs_info, struct bio *bio,
 	map_length = length;
 
 	btrfs_bio_counter_inc_blocked(fs_info);
-	ret = __btrfs_map_block(fs_info, bio_op(bio), logical,
+	ret = __btrfs_map_block(fs_info, btrfs_op(bio), logical,
 				&map_length, &bbio, mirror_num, 1);
 	if (ret) {
 		btrfs_bio_counter_dec(fs_info);