f2fs: fix the periodic wakeups of discard thread

Fix the unnecessary periodic wakeups of discard thread that happens under
below two conditions -

1. When f2fs is heavily utilized over 80%, the current discard policy
sets the max sleep timeout of discard thread as 50ms
(DEF_MIN_DISCARD_ISSUE_TIME). But this is set even when there are
no pending discard commands to be issued.

2. In the issue_discard_thread() path when there are no pending discard
commands, it fails to reset the wait_ms to max timeout value.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>