vxlan driver should be using helpers to access nexthop struct
internals. Remove open check if whether nexthop is multipath in
favor of the existing nexthop_is_multipath helper. Add a new
helper, nexthop_has_v4, to cover the need to check has_v4 in
a group.

Fixes: 1274e1cc ("vxlan: ecmp support for mac fdb entries")
Cc: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>