Fix parsing of IPv6 addresses with peer from netlink.

In the most common case, an IPv6 address doesn't have a peer and the
IFA_ADDRESS netlink attribute contains the address itself.

But if the address has a peer (typically for point to point links),
then IFA_ADDRESS contains the peer address and IFA_LOCAL contains the
address [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/addrconf.c?h=v5.17#n5030

Fix the parsing of IPv6 addresses with peers, as currently dnsmasq
unsuccessfully tries to bind on the peer address.

A simple reproducer is:

  dnsmasq --conf-file=/dev/null -i dummy1 -d --bind-dynamic &
  sleep 2
  ip link add dummy1 type dummy
  ip link set dummy1 up
  ip addr add dev dummy1 fd01::1/64 peer fd01::2/64
  ip addr add dev dummy1 fd01::42/64
  sleep 2
  ss -lnp | grep dnsmasq | grep fd01

Before the patch:
  dnsmasq: failed to create listening socket for fd01::2: Cannot assign requested address
  dnsmasq: failed to create listening socket for fd01::2: Cannot assign requested address
  udp   UNCONN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23947,fd=14))
  tcp   LISTEN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23947,fd=15

After:
  udp   UNCONN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=16))
  udp   UNCONN 0    [fd01::1]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=14))
  tcp   LISTEN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=17))
  tcp   LISTEN 0    [fd01::1]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=15))

src/netlink.c

@@ -258,7 +258,16 @@ int iface_enumerate(int family, void *parm, int (*callback)())
 
 		    while (RTA_OK(rta, len1))
 		      {
-			if (rta->rta_type == IFA_ADDRESS)
+			/*
+			 * Important comment: (from if_addr.h)
+			 * IFA_ADDRESS is prefix address, rather than local interface address.
+			 * It makes no difference for normally configured broadcast interfaces,
+			 * but for point-to-point IFA_ADDRESS is DESTINATION address,
+			 * local address is supplied in IFA_LOCAL attribute.
+			 */
+			if (rta->rta_type == IFA_LOCAL)
+			  addrp = ((struct in6_addr *)(rta+1));
+			else if (rta->rta_type == IFA_ADDRESS && !addrp)
 			  addrp = ((struct in6_addr *)(rta+1)); 
 			else if (rta->rta_type == IFA_CACHEINFO)
 			  {