fsck.f2fs: update superblock if invalid

If a superblock failed in sanity check, it should be fixed. This patch
add a new state `sb_invalid' to tell fsck needs to update superblock
at the end of all checkings.

This patch also cleans up force_stop, abnormal_stop, fs_errors and
sb_invalid by merging them into an `invalid_sb' flags, and each of
them is indicated using one bit.

Signed-off-by: Sheng Yong <shengyong@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>