netlink: Fix build with mmap disabled.

net/netlink/diag.c: In function 'sk_diag_put_rings_cfg':
net/netlink/diag.c:28:17: error: 'struct netlink_sock' has no member named 'pg_vec_lock'
net/netlink/diag.c:29:29: error: 'struct netlink_sock' has no member named 'rx_ring'
net/netlink/diag.c:31:30: error: 'struct netlink_sock' has no member named 'tx_ring'
net/netlink/diag.c:33:19: error: 'struct netlink_sock' has no member named 'pg_vec_lock'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>