f2fs: don't call f2fs_issue_discard_timeout() when discard_cmd_cnt is 0 in f2fs_put_super()

No need to call f2fs_issue_discard_timeout() in f2fs_put_super,
when no discard command requires issue. Since the caller of
f2fs_issue_discard_timeout() usually judges the number of discard
commands before using it. Let's move this logic to
f2fs_issue_discard_timeout().

By the way, use f2fs_realtime_discard_enable to simplify the code.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>