mlxsw: spectrum_router: Align multipath hash parameters with kernel's

Up until now we used the hardware's defaults for multipath hash
computation. This patch aligns the hardware's multipath parameters with
the kernel's.

For IPv4 packets, the parameters are determined according to the
'fib_multipath_hash_policy' sysctl during module initialization. In case
L3-mode is requested, only the source and destination IP addresses are
used. There is no special handling of ICMP error packets.

In case L4-mode is requested, a 5-tuple is used: source and destination
IP addresses, source and destination ports and IP protocol. Note that
the layer 4 fields are not considered for fragmented packets.

For IPv6 packets, the source and destination IP addresses are used, as
well as the flow label and the next header fields.

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