dummynet: fix sysctls

The sysctl nodes which use V_dn_cfg must be marked as CTLFLAG_VNET so
that we use the correct per-vnet offset

PR: 256819
Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30974