rtnetlink: Add RTNH_F_TRAP flag

The flag indicates to user space that the nexthop is not programmed to
forward packets in hardware, but rather to trap them to the CPU. This is
needed, for example, when the MAC of the nexthop neighbour is not
resolved and packets should reach the CPU to trigger neighbour
resolution.

The flag will be used in subsequent patches by netdevsim to test nexthop
objects programming to device drivers and in the future by mlxsw as
well.

Changes since RFC:
* Reword commit message

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>