btrfs: be a bit more careful when setting mirror_num_ret in btrfs_map_block

The mirror_num_ret is allowed to be NULL, although it has to be set when
smap is set.  Unfortunately that is not a well enough specifiable
invariant for static type checkers, so add a NULL check to make sure they
are fine.

Fixes: 03793cb ("btrfs: add fast path for single device io in __btrfs_map_block")
Reported-by: Dan Carpenter <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: Christoph Hellwig

fs/btrfs/volumes.c

@@ -6398,7 +6398,8 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	    (op == BTRFS_MAP_READ || !dev_replace_is_ongoing ||
 	     !dev_replace->tgtdev)) {
 		set_io_stripe(smap, map, stripe_index, stripe_offset, stripe_nr);
-		*mirror_num_ret = mirror_num;
+		if (mirror_num_ret)
+			*mirror_num_ret = mirror_num;
 		*bioc_ret = NULL;
 		ret = 0;
 		goto out;