mpls: make via address optional for multipath routes

The via address is optional for a single path route, yet is mandatory
when the multipath attribute is used:

  # ip -f mpls route add 100 dev lo
  # ip -f mpls route add 101 nexthop dev lo
  RTNETLINK answers: Invalid argument

Make them consistent by making the via address optional when the
RTA_MULTIPATH attribute is being parsed so that both forms of
specifying the route work.

Signed-off-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>