net: don't panic on ifconfig pfsync0 mtu 9000

pfsync interfaces do not have ifp->if_inet6 set, so when we update the
MTU for those interfaces we panicked.

Add an explicit check for this. This should be temporary, until pfsync
is no longer a struct ifnet (as we've already done for pflog).

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D58701