net: mctp: Don't access ifa_index when missing

In the Linux kernel, the following vulnerability has been resolved:

net: mctp: Don't access ifa_index when missing

In mctp_dump_addrinfo, ifa_index can be used to filter interfaces, but

only when the struct ifaddrmsg is provided. Otherwise it will be

comparing to uninitialised memory - reproducible in the syzkaller case from

dhcpd, or busybox "ip addr show".

The kernel MCTP implementation has always filtered by ifa_index, so

existing userspace programs expecting to dump MCTP addresses must

already be passing a valid ifa_index value (either 0 or a real index).

BUG: KMSAN: uninit-value in mctp_dump_addrinfo+0x208/0xac0 net/mctp/device.c:128

mctp_dump_addrinfo+0x208/0xac0 net/mctp/device.c:128

rtnl_dump_all+0x3ec/0x5b0 net/core/rtnetlink.c:4380

rtnl_dumpit+0xd5/0x2f0 net/core/rtnetlink.c:6824

netlink_dump+0x97b/0x1690 net/netlink/af_netlink.c:2309