fsck.f2fs: add auto_fix feature

fsck.f2fs: add auto_fix feature

This patch adds an auto_fix feature which fixes inconsistency of f2fs images.

E.g.,

With this option, -a, fsck.f2fs tries to fix inconsistency only if its valid
checkpoint has CP_FSCK_FLAG, written by previous bug_on cases.

So, normally it does fix nothing, so that there is no performance regression.
But, if a sort of corruption was reported by the f2fs module, this tries to
fix potential corrupted partition.

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