net: ipv6: Introduce ip6_multipath_hash_policy()

net: ipv6: Introduce ip6_multipath_hash_policy()

In order to abstract away access to the
ipv6.sysctl.multipath_hash_policy variable, which is not available on
systems compiled without IPv6 support, introduce a wrapper function
ip6_multipath_hash_policy() that falls back to 0 on non-IPv6 systems.

Use this wrapper from mlxsw/spectrum_router instead of a direct
reference.

Signed-off-by: Petr Machata <[email protected]>
Signed-off-by: Ido Schimmel <[email protected]>
Acked-by: David Ahern <[email protected]>
Signed-off-by: David S. Miller <[email protected]>