pf(4) needs to have a correct checksum during its processing.
Calculate checksums for the IPv6 path when needed before
delving into pf(4) code as required.

PR:     172648, 179392
Reviewed by:    glebius@
Approved by:    gnn@
Obtained from:  pfSense
MFC after:      1 week
Sponsored by:   Netgate


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274709 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f