ip: nexthop: add cache helpers

ip: nexthop: add cache helpers

Add a static nexthop cache in a hash with 1024 buckets and helpers to
manage it (link, unlink, find, add nexthop, del nexthop). Adding new
nexthops is done by creating a new rtnl handle and using it to retrieve
the nexthop so the helper is safe to use while already reading a
response (i.e. using the global rth).

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>