net, rps: fix build failure when CONFIG_RPS isn't set

In file included from net/socket.c:99:0:
include/net/sock.h: In function ‘sock_rps_record_flow’:
include/net/sock.h:849:30: error: ‘const struct sock’ has no member named ‘sk_rxhash’
include/net/sock.h: In function ‘sock_rps_reset_flow’:
include/net/sock.h:854:29: error: ‘const struct sock’ has no member named ‘sk_rxhash’

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>