f2fs: introduce __check_sit_bitmap

After we introduce discard thread, discard command can be issued
concurrently with data allocating, this patch adds new function to
heck sit bitmap to ensure that userdata was invalid in which on-going
discard command covered.

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