btrfs: raid56: properly handle the error when unable to find the missing stripe

In raid56_alloc_missing_rbio(), if we can not determine where the
missing device is inside the full stripe, we just BUG_ON().

This is not necessary especially the only caller inside scrub.c is
already properly checking the return value, and will treat it as a
memory allocation failure.

Fix the error handling by:

- Add an extra warning for the reason
  Although personally speaking it may be better to be an ASSERT().

- Properly free the allocated rbio
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -2742,8 +2742,10 @@ raid56_alloc_missing_rbio(struct bio *bio, struct btrfs_io_context *bioc)
 
 	rbio->faila = find_logical_bio_stripe(rbio, bio);
 	if (rbio->faila == -1) {
-		BUG();
-		kfree(rbio);
+		btrfs_warn_rl(fs_info,
+	"can not determine the failed stripe number for full stripe %llu",
+			      bioc->raid_map[0]);
+		__free_raid_bio(rbio);
 		return NULL;
 	}