mlxsw: spectrum_router: Store nexthop groups in a hash table

Currently, when we're notified about a new RTN_UNICAST route we perform
a lookup on the nexthop group list looking for a group with a matching
configuration to that found in the FIB info. This is quite inefficient.

Instead, we can simply rely on the kernel to consolidate several FIB
configurations into the same FIB info and use the FIB info as the key
for our private nexthop group struct.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>