netlink: specs: Add route flow label attribute

Add the new flow label attribute to the spec. Example:

 # ip link add name dummy1 up type dummy
 # ip -6 route add default table 254 dev dummy1
 # ip -6 route add default table 10 dev dummy1
 # ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
	--do newrule \
	--json '{"family": 10, "priority": 1, "flowlabel": 10, "flowlabel-mask": 255, "action": 1, "table": 10}'
 None
 $ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_route.yaml \
	--do getroute \
	--json '{"rtm-family": 10, "rta-flowlabel": 1}' --output-json \
	| jq '.["rta-table"]'
 254
 $ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_route.yaml \
	--do getroute \
	--json '{"rtm-family": 10, "rta-flowlabel": 10}' --output-json \
	| jq '.["rta-table"]'
 10

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>