f2fs: check in-memory nat version bitmap

commit 599a09b2c1ac222e6aad0c22515d1ccde7c3b702 upstream.

This patch adds a mirror for nat 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>