netlink: specs: Add FIB rule flow label attributes

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

 # ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
	--do newrule \
	--json '{"family": 10, "flowlabel": 1, "flowlabel-mask": 1, "action": 1, "table": 1}'
 None
 $ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
	--dump getrule --json '{"family": 10}' --output-json \
	| jq '.[] | select(.flowlabel == "0x1")'
 {
   "table": 1,
   "suppress-prefixlen": "0xffffffff",
   "protocol": 0,
   "priority": 32765,
   "flowlabel": "0x1",
   "flowlabel-mask": "0x1",
   "family": 10,
   "dst-len": 0,
   "src-len": 0,
   "tos": 0,
   "action": "to-tbl",
   "flags": 0
 }

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>