Btrfs: do not BUG_ON on aborted situation

Btrfs balance can easily hit BUG_ON in these places, but we want
to it bail out gracefully after we force the whole filesystem to
readonly.  So we use btrfs_std_error hook in place of BUG_ON.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/relocation.c

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 9d13786..3ebe8797 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3771,7 +3771,11 @@
 	while (1) {
 		progress++;
 		trans = btrfs_start_transaction(rc->extent_root, 0);
-		BUG_ON(IS_ERR(trans));
+		if (IS_ERR(trans)) {
+			err = PTR_ERR(trans);
+			trans = NULL;
+			break;
+		}
 restart:
 		if (update_backref_cache(trans, &rc->backref_cache)) {
 			btrfs_end_transaction(trans, rc->extent_root);