Fix TSO support on VMware Fusion

Apparently for VMware Fusion (and presumably VMware Workstation/Player
since the PR states TSO is broken there too, but I cannot test), the
TCP header pseudo checksum calculated should only include the protocol
(IPPROTO_TCP) value, not also the lengths as the stack does instead.

VMware ESXi seems to ignore whatever value is in the TCP header checksum,
and it is a bit surprising there is a different behavior between the
VMware products. And it is unfortunate that on ESXi we are forced to do
this extra bit of work.

PR:		kern/185849
MFC after:	3 days