Move ip_ipsec_fwd() from ip_input() into ip_forward().

Remove check for presence PACKET_TAG_IPSEC_IN_DONE mbuf tag from
ip_ipsec_fwd(). PACKET_TAG_IPSEC_IN_DONE tag means that packet is
already handled by IPSEC code. This means that before IPSEC processing
it was destined to our address and security policy was checked in
the ip_ipsec_input(). After IPSEC processing packet has new IP
addresses and destination address isn't our own. So, anyway we can't
check security policy from the mbuf tag, because it corresponds
to different addresses.

We should check security policy that corresponds to packet
attributes in both cases - when it has a mbuf tag and when it has not.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC
Remove unused mtag variable.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC
Remove code related to PACKET_TAG_IPSEC_IN_CRYPTO_DONE mbuf tag.
It isn't used in FreeBSD.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC
Remove now unused mtag argument from ipsec*_common_input_cb.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC