f2fs: trace __submit_discard_cmd

Add an even class f2fs_discard for introducing f2fs_queue_discard, then
use f2fs_{queue,issue}_discard to trace __{queue,submit}_discard_cmd.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>