btrfs: check if the transaction was aborted at btrfs_wait_for_commit()

At btrfs_wait_for_commit() we wait for a transaction to finish and then
always return 0 (success) without checking if it was aborted, in which
case the transaction didn't happen due to some critical error. Fix this
by checking if the transaction was aborted.

Fixes: 4620459 ("Btrfs: add START_SYNC, WAIT_SYNC ioctls")
CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/transaction.c

@@ -931,6 +931,7 @@ int btrfs_wait_for_commit(struct btrfs_fs_info *fs_info, u64 transid)
 	}
 
 	wait_for_commit(cur_trans, TRANS_STATE_COMPLETED);
+	ret = cur_trans->aborted;
 	btrfs_put_transaction(cur_trans);
 out:
 	return ret;