ubifs: Fix missing error code err

Fix smatch warning:

fs/ubifs/journal.c:1610 ubifs_jnl_truncate() warn: missing error code
'err'

Signed-off-by: Ferry Meng <mengferry@linux.alibaba.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>