f2fs: fix use-after-free of dicard command entry

As Dan Carpenter reported:

The patch 20ee4382: "f2fs: issue small discard by LBA order" from
Jul 8, 2018, leads to the following Smatch warning:

	fs/f2fs/segment.c:1277 __issue_discard_cmd_orderly()
	warn: 'dc' was already freed.

See also:
fs/f2fs/segment.c:2550 __issue_discard_cmd_range() warn: 'dc' was already freed.

In order to fix this issue, let's get error from __submit_discard_cmd(),
and release current discard command after we referenced next one.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>