mpls: validate L2 via address length

mpls: validate L2 via address length

If an L2 via address for an mpls nexthop is specified, the length of
the L2 address must match that expected by the output device,
otherwise it could access memory beyond the end of the via address
buffer in the route.

This check was present prior to commit 660b8cb11208 ("mpls: multipath
route support"), but got lost in the refactoring, so add it back,
applying it to all nexthops in multipath routes.

Fixes: 660b8cb11208 ("mpls: multipath route support")
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>