udp: implement GRO for plain UDP sockets.

This is the RX counterpart of commit bec1f6f697 ("udp: generate gso
with UDP_SEGMENT"). When UDP_GRO is enabled, such socket is also
eligible for GRO in the rx path: UDP segments directed to such socket
are assembled into a larger GSO_UDP_L4 packet.

The core UDP GRO support is enabled with setsockopt(UDP_GRO).

Initial benchmark numbers:

Before:
udp rx:   1079 MB/s   769065 calls/s

After:
udp rx:   1466 MB/s    24877 calls/s

This change introduces a side effect in respect to UDP tunnels:
after a UDP tunnel creation, now the kernel performs a lookup per ingress
UDP packet, while before such lookup happened only if the ingress packet
carried a valid internal header csum.

rfc v2 -> rfc v3:
 - fixed typos in macro name and comments
 - really enforce UDP_GRO_CNT_MAX, instead of UDP_GRO_CNT_MAX + 1
 - acquire socket lock in UDP_GRO setsockopt

rfc v1 -> rfc v2:
 - use a new option to enable UDP GRO
 - use static keys to protect the UDP GRO socket lookup

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -2473,6 +2473,14 @@ int udp_lib_setsockopt(struct sock *sk, int level, int optname,
 		up->gso_size = val;
 		break;
 
+	case UDP_GRO:
+		lock_sock(sk);
+		if (valbool)
+			udp_tunnel_encap_enable(sk->sk_socket);
+		up->gro_enabled = valbool;
+		release_sock(sk);
+		break;
+
 	/*
 	 * 	UDP-Lite's partial checksum coverage (RFC 3828).
 	 */