Handle delayed checksums if needed in NAT64.

Upper level protocols defer checksums calculation in hope we have
checksums offloading in a network card. CSUM_DELAY_DATA flag is used
to determine that checksum calculation was deferred. And IP output
routine checks for this flag before pass mbuf to lower layer. Forwarded
packets have not this flag.

NAT64 uses checksums adjustment when it translates IP headers.
In most cases NAT64 is used for forwarded packets, but in case when it
handles locally originated packets we need to finish checksum calculation
that was deferred to correctly adjust it.

Add check for presence of CSUM_DELAY_DATA flag and finish checksum
calculation before adjustment.

Reported and tested by: Evgeniy Khramtsov <evgeniy at khramtsov org>
MFC after: 1 week