sbin/sysctl: Fix CTLFLAG_SKIP for adjacent nodes

The OID is saved when we encounter CTLFLAG_SKIP so that descendants can
be skipped as well. We then must not update the skip OID until we are
out of the node. This was achieved by resetting the skip OID once the
prefix no longer matches, but the case where the OID we reset on has
CTLFLAG_SKIP was not accounted for.

Reported by: mav
Reviewed by: mav
MFC after: 2 days
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D28364