Netlink: Add option to specify netlink socket receive buffer size

Add option 'netlink rx buffer' to specify netlink socket receive buffer
size. Uses SO_RCVBUFFORCE, so it can override rmem_max limit.

Thanks to Trisha Biswas and Michal for the original patches.

sysdep/linux/krt-sys.h

@@ -69,6 +69,7 @@ static inline struct ifa * kif_get_primary_ip(struct iface *i UNUSED) { return N
 struct krt_params {
   u32 table_id;				/* Kernel table ID we sync with */
   u32 metric;				/* Kernel metric used for all routes */
+  uint netlink_rx_buffer;		/* Rx buffer size for the netlink socket */
 };
 
 struct krt_state {