pty04: Avoid receiving packets from all interfaces

Raw sockets receive all packets from all interfaces as soon as they are
created. They don't need to be bound to one or more interfaces like I thought.

This creates the socket with a NULL packet type which appears to be unused in
the Linux kernel and receives no packets. The protocol is then changed to
ETH_P_ALL when binding to the netdevice we are interested in.

I assume it is impossible for a packet to have zero for its packet type. Most
likely using ETH_P_LOOP would have been OK too, but I suppose that is really
used by some hardware.

This also fixes various other problems that have been observed with this
test. Such as memory allocation failures.

Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
Cc: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
Tested-by: Jan Stancek <jstancek@redhat.com>

Author: Richard Palethorpe

testcases/kernel/pty/pty04.c

@@ -206,7 +206,7 @@ static void open_netdev(const struct ldisc_info *ldisc)
 	SAFE_IOCTL(pts, SIOCGIFNAME, ifreq.ifr_name);
 	tst_res(TINFO, "Netdev is %s", ifreq.ifr_name);
 
-	sk = SAFE_SOCKET(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
+	sk = SAFE_SOCKET(PF_PACKET, SOCK_RAW, 0);
 
 	ifreq.ifr_mtu = ldisc->mtu;
 	if (ioctl(sk, SIOCSIFMTU, &ifreq))