pf: disallow IPv6 routing header by default

pf drops IPv4 packets with any options by default. For IPv6 the
same is already done for options header. Add the routing extension
header to the list that need "allow-opts" to pass.
OK sashan@ visa@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, bfcbb272c6
Sponsored by: Rubicon Communications, LLC ("Netgate")