Remove bogus check for accept queue length and associated failure handling
from the incoming SYN handling section of tcp_input().

Enforcement of the accept queue limits is done by sonewconn() after the
3WHS is completed. It is not necessary to have an earlier check before a
connection request enters the SYN cache awaiting the full handshake. It
rather limits the effectiveness of the syncache by preventing legit and
illegit connections from entering it and having them shaken out before we
hit the real limit which may have vanished by then.

Change return value of syncache_add() to void. No status communication
is required.