btrfs: handle errors while updating refcounts in update_ref_for_cow

Since commit fb235dc06fa (btrfs: qgroup: Move half of the qgroup
accounting time out of commit trans) the assumption that
btrfs_add_delayed_{data,tree}_ref can only return 0 or -ENOMEM has
been false. The qgroup operations call into btrfs_search_slot
and friends and can now return the full spectrum of error codes.

Fortunately, the fix here is easy since update_ref_for_cow failing
is already handled so we just need to bail early with the error
code.

Fixes: fb235dc06fa (btrfs: qgroup: Move half of the qgroup accounting ...)
Cc: <[e-mail]> # v4.11+
Signed-off-by: Jeff Mahoney <[e-mail]>
Reviewed-by: Edmund Nadolski <[e-mail]>
Reviewed-by: Qu Wenruo <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/ctree.c

···
 		     root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID) &&
 		    !(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF)) {
 			ret = btrfs_inc_ref(trans, root, buf, 1);
-			BUG_ON(ret); /* -ENOMEM */
+			if (ret)
+				return ret;
 
 			if (root->root_key.objectid ==
 			    BTRFS_TREE_RELOC_OBJECTID) {
 				ret = btrfs_dec_ref(trans, root, buf, 0);
-				BUG_ON(ret); /* -ENOMEM */
+				if (ret)
+					return ret;
 				ret = btrfs_inc_ref(trans, root, cow, 1);
-				BUG_ON(ret); /* -ENOMEM */
+				if (ret)
+					return ret;
 			}
 			new_flags |= BTRFS_BLOCK_FLAG_FULL_BACKREF;
 		} else {
···
 				ret = btrfs_inc_ref(trans, root, cow, 1);
 			else
 				ret = btrfs_inc_ref(trans, root, cow, 0);
-			BUG_ON(ret); /* -ENOMEM */
+			if (ret)
+				return ret;
 		}
 		if (new_flags != 0) {
 			int level = btrfs_header_level(buf);
···
 				ret = btrfs_inc_ref(trans, root, cow, 1);
 			else
 				ret = btrfs_inc_ref(trans, root, cow, 0);
-			BUG_ON(ret); /* -ENOMEM */
+			if (ret)
+				return ret;
 			ret = btrfs_dec_ref(trans, root, buf, 1);
-			BUG_ON(ret); /* -ENOMEM */
+			if (ret)
+				return ret;
 		}
 		clean_tree_block(fs_info, buf);
 		*last_ref = 1;