MFC r290161:

MFC r290161:

pf: Fix IPv6 checksums with route-to.

When using route-to (or reply-to) pf sends the packet directly to the output
interface. If that interface doesn't support checksum offloading the checksum
has to be calculated in software.
That was already done in the IPv4 case, but not for the IPv6 case. As a result
we'd emit packets with pseudo-header checksums (i.e. incorrect checksums).

This issue was exposed by the changes in r289316 when pf stopped performing full
checksum calculations for all packets.

Submitted by:       Luoqi Chen

git-svn-id: svn://svn.freebsd.org/base/stable/10@290669 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f