f2fs: don't issue discard commands in online discard is on

Actually, we don't need to issue discard commands, if discard is on, as
mentioned in the comment.

Change-Id: Ia806b1d700f5fea91e399a80ba9012676cc1271e
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Git-commit: 0d71db20
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>