net: guard tcp_set_keepalive() to tcp sockets

Its possible to use RAW sockets to get a crash in
tcp_set_keepalive() / sk_reset_timer()

Fix is to make sure socket is a SOCK_STREAM one.

Change-Id: Ieeb498a3e623cfcb54e1c865a3c0229e4acf1e87
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>