nexthop: Add ability to specify group type

nexthop: Add ability to specify group type

Next patches are going to add a 'resilient' nexthop group type, so allow
users to specify the type using the 'type' argument. Currently, only
'mpath' type is supported.

These two commands are equivalent:

 # ip nexthop add id 10 group 1/2/3
 # ip nexthop add id 10 group 1/2/3 type mpath

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>