Make fsck_ffs more persistent in creating a lost+found directory.

When fsck_ffs is running in interactive mode and finds unlinked files,
it offers to either unlink them or place them in a lost+found directory.
If the lost+found directory option is requested and no lost+found
directory exists, fsck_ffs offers to create one. When creating one,
it must allocate an inode and a filesystem block. It attempts to
allocate them from the first cylinder group. If the first cylinder
group has a bad check hash, it gives up.

This change expands the search into later cylinder groups when the
first one fails with a bad check hash.

Reported by: Chuck Silvers
Tested by: Chuck Silvers
MFC after: 1 week
Sponsored by: Netflix