mpls: Properly validate RTA_VIA payload length

If the nla length is less than 2 then the nla data could be accessed
beyond the accessible bounds. So ensure that the nla is big enough to
at least read the via_family before doing so. Replace magic value of
2.

Fixes: 03c05665 ("mpls: Basic support for adding and removing routes")
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>