rxrpc: Fix network address validation

Fix network address validation on entry to uapi functions such as connect()
for AF_RXRPC.  The check for address compatibility with the transport
socket isn't correct and allows an AF_INET6 address to be given to an
AF_INET socket, resulting in an oops now that rxrpc is calling
udp_sendmsg() directly.

Sample program:

	#define _GNU_SOURCE
	#include <stdio.h>
	#include <stdlib.h>
	#include <sys/socket.h>
	#include <arpa/inet.h>
	#include <linux/rxrpc.h>
	static unsigned char ctrl[256] =
		"\x18\x00\x00\x00\x00\x00\x00\x00\x10\x01\x00\x00\x01";
	int main(void)
	{
		struct sockaddr_rxrpc srx = {
			.srx_family			= AF_RXRPC,
			.transport_type			= SOCK_DGRAM,
			.transport_len			= 28,
			.transport.sin6.sin6_family	= AF_INET6,
		};
		struct mmsghdr vec = {
			.msg_hdr.msg_control	= ctrl,
			.msg_hdr.msg_controllen	= 0x18,
		};
		int s;
		s = socket(AF_RXRPC, SOCK_DGRAM, AF_INET);
		if (s < 0) {
			perror("socket");
			exit(1);
		}
		if (connect(s, (struct sockaddr *)&srx, sizeof(srx)) < 0) {
			perror("connect");
			exit(1);
		}
		if (sendmmsg(s, &vec, 1, MSG_NOSIGNAL | MSG_MORE) < 0) {
			perror("sendmmsg");
			exit(1);
		}
		return 0;
	}

If working properly, connect() should fail with EAFNOSUPPORT.

Fixes: ed472b0c ("rxrpc: Call udp_sendmsg() directly")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org