f2fs: check in-memory sit version bitmap

commit ae27d62e6befd3cac4ffa702e644cc52019642e8 upstream.

This patch adds a mirror for sit version bitmap, and use it to detect
in-memory bitmap corruption which may be caused by bit-transition of
cache or memory overflow.

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