udp: Introduce recvmmsg()/sendmmsg(), zero-copy path from socket

Packets are received directly onto pre-cooked, static buffers
for IPv4 (with partial checksum pre-calculation) and IPv6 frames,
with pre-filled Ethernet addresses and, partially, IP headers,
and sent out from the same buffers with sendmmsg(), for both
passt and pasta (non-local traffic only) modes.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

udp.h

@@ -9,6 +9,8 @@ int udp_tap_handler(struct ctx *c, int af, void *addr,
 		    struct tap_msg *msg, int count, struct timespec *now);
 int udp_sock_init(struct ctx *c);
 void udp_timer(struct ctx *c, struct timespec *ts);
+void udp_update_l2_buf(unsigned char *eth_d, unsigned char *eth_s,
+		       uint32_t *ip_da);
 
 /**
  * union udp_epoll_ref - epoll reference portion for TCP connections