[TCP]: Replace socket with sock for reset sending.

Replace tcp_socket with tcp_sock. This is more effective (less
derefferences on fast paths). Additionally, the approach is unified to
one used in ICMP.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>