In tcp_input(), don't acquire the pcbinfo global write lock for SYN
packets targeting a listening socket. Permit to reduce TCP input
processing starvation in context of high SYN load (e.g. short-lived TCP
connections or SYN flood).

Submitted by: Julien Charbon <jcharbon@verisign.com>
Reviewed by: adrian, hiren, jhb, Mike Bentkofsky