Revert "f2fs: rebuild nat_bits during umount"

This reverts commit 94c821fb.

It reports that there is potential corruption in node footer,
the most suspious feature is nat_bits, let's revert recovery
related code.

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