Do not panic in case of corrupted UFS/FFS directory.

Historically the system panic'ed when it encountered a corrupt
directory. This change recovers well enough to continue operations.
This change is made in response to a similar change made in the ext2
filesystem as described in the cited Differential Revision.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38503