nexthop: Allow setting "offload" and "trap" indications on nexthops

Add a function that can be called by device drivers to set "offload" or
"trap" indication on nexthops following nexthop notifications.

Changes since RFC:
* s/nexthop_hw_flags_set/nexthop_set_hw_flags/

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>