nexthop: Specialize rtm_nh_policy

This policy is currently only used for creation of new next hops and new
next hop groups. Rename it accordingly and remove the two attributes that
are not valid in that context: NHA_GROUPS and NHA_MASTER.

For consistency with other policies, do not mention policy array size in
the declarator, and replace NHA_MAX for ARRAY_SIZE as appropriate.

Note that with this commit, NHA_MAX and __NHA_MAX are not used anymore.
Leave them in purely as a user API.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>