net: don't panic on ifconfig pfsync0 mtu 9000

[kp]

Summary

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).

Sponsored by: Rubicon Communications, LLC ("Netgate")

[glebius]

sys/net/route.c

519–521

Let's mark it with IFT_PFSYNC so that won't be forgotten to be removed.

[kp]

Add IFT_PFSYNC comment.