FROMGIT: f2fs: skip adding a discard command if exists

When recovering zoned UFS, sometimes we add the same zone to discard multiple
times. Simple workaround is to bypass adding it.

Reviewed-by: Chao Yu <chao@kernel.org>
Change-Id: Ic1501caa02aea8bdc9d7f00a8bd0d35be2ef4ba1
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit bbd3efed
 https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)