Read IPsec forwarding information once.

Read IPsec forwarding information once.

Fix MP race between reading ip_forwarding in ip_input() and checking
ip_forwarding == 2 in ip_output().  In theory ip_forwarding could
be 2 during ip_input() and later 0 in ip_output().  Then a packet
would be forwarded that was never allowed.  Currently exclusive
netlock in sysctl(2) prevents all races.

Introduce IP_FORWARDING_IPSEC and pass it with the flags parameter
that was introduced for IP_FORWARDING.

Instead of calling m_tag_find(), traversing the list, and comparing
with NULL, just check the PACKET_TAG_IPSEC_IN_DONE bit.  Reading
ipsec_in_use in ip_output() is a performance hack that is not
necessary.  New code only checks tree bits.

OK mvs@