vxlan: Remove access to nexthop group struct

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>