Cleanups to fsck_ffs(8).

When checking an inode ensure that it does not have a negative size.
Stop scaning a directory when an unallocated block is found.
Fully clear an inode when it is first allocated.
Ensure that an inode is marked dirty whenever it is updated and that
it has a correct check hash when it is released.

MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation