Remove the support for mknod(S_IFMT), which created dummy vnodes with
VBAD type.

FFS ffs_write() VOP catches such vnodes and panics, other VOPs do not
check for the type and their behaviour is really undefined. The
comment claims that this support was done for 'badsect' to flag bad
sectors, we do not have such facility in kernel anyway.

Reported by: Dmitry Vyukov <dvyukov@google.com>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week