ipv6: Add flow label to route get requests

The default IPv6 multipath hash policy takes the flow label into account
when calculating a multipath hash and previous patches added a flow
label selector to IPv6 FIB rules.

Allow user space to specify a flow label in route get requests by adding
a new netlink attribute and using its value to populate the "flowlabel"
field in the IPv6 flow info structure prior to a route lookup.

Deny the attribute in RTM_{NEW,DEL}ROUTE requests by checking for it in
rtm_to_fib6_config() and returning an error if present.

A subsequent patch will use this capability to test the new flow label
selector in IPv6 FIB rules.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>