MFC/backport core kernel and userspace parts of r237263 (TCP_OFFLOAD
rework).  MFC r237563, r239511, r243603, r245915, r245916, r245919,
r245921, r245922, r245924, r245925, r245932, r245934 too.

Build tested with make universe.

r237263:
- Updated TOE support in the kernel.
...

r237563:
Fix clang warning when compiling iw_cxgb.

r239511:
Correctly handle the case where an inp has already been dropped by the time
the TOE driver reports that an active open failed.  toe_connect_failed is
supposed to handle this but it should be provided the inpcb instead of the
tcpcb which may no longer be around.

r243603:
Make sure that tcp_timer_activate() correctly sees TCP_OFFLOAD (or not).

r245915:
Heed SO_NO_OFFLOAD.

r245916:
Teach toe_4tuple_check() to deal with IPv6 4-tuples too.

r245919:
Add TCP_OFFLOAD hook in syncache_respond for IPv6 too, just like the one
that exists for IPv4.

r245921:
There is no need to call into the TOE driver twice in pru_rcvd (tod_rcvd
and then tod_output right after that).

r245922:
Avoid NULL dereference in nd6_storelladdr when no mbuf is provided.  It
is called this way from a couple of places in the OFED code.  (toecore
calls it too but that's going to change shortly).

r245924:
Move lle_event to if_llatbl.h

lle_event replaced arp_update_event after the ARP rewrite and ended up
in if_ether.h simply because arp_update_event used to be there too.
IPv6 neighbor discovery is going to grow lle_event support and this is a
good time to move it to if_llatbl.h.

The two in-tree consumers of this event - OFED and toecore - are not
affected.

r245925:
Generate lle_event in the IPv6 neighbor discovery code too.

r245932:
Teach toe_l2_resolve to resolve IPv6 destinations too.

r245934:
Add checks for SO_NO_OFFLOAD in a couple of places that I missed earlier
in r245915.