f2fs: fix to avoid long latency during umount

In umount, we give an constand time to handle pending discard, previously,
in __issue_discard_cmd() we missed to check timeout condition in loop,
result in delaying long time, fix it.

Signed-off-by: Heng Xiao <heng.xiao@unisoc.com>
[Chao Yu: add commit message]
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>