Do some re-sorting of TCP pcbinfo locking and assertions: make sure to

Do some re-sorting of TCP pcbinfo locking and assertions: make sure to
retain the pcbinfo lock until we're done using a pcb in the in-bound
path, as the pcbinfo lock acts as a pseuo-reference to prevent the pcb
from potentially being recycled.  Clean up assertions and make sure to
assert that the pcbinfo is locked at the head of code subsections where
it is needed.  Free the mbuf at the end of tcp_input after releasing
any held locks to reduce the time the locks are held.

MFC after:	3 weeks