tools: ynl: add a sample for rt-link

tools: ynl: add a sample for rt-link

Add a fairly complete example of rt-link usage. If run without any
arguments it simply lists the interfaces and some of their attrs.
If run with an arg it tries to create and delete a netkit device.

 1 # ./tools/net/ynl/samples/rt-link 1
 2 Trying to create a Netkit interface
 3 Testing error message for policy being bad:
 4     Kernel error: 'Provided default xmit policy not supported' (bad attribute: .linkinfo.data(netkit).policy)
 5   1:               lo: mtu 65536
 6   2:           wlp0s1: mtu  1500
 7   3:          enp0s13: mtu  1500
 8   4:           dummy0: mtu  1500  kind dummy     altname one two
 9   5:              nk0: mtu  1500  kind netkit    primary 0  policy forward
10   6:              nk1: mtu  1500  kind netkit    primary 1  policy blackhole
11 Trying to delete a Netkit interface (ifindex 6)

Sample creates the device first, it sets an invalid value for a netkit
attribute to trigger reverse parsing. Line 4 shows the error with the
attribute path correctly generated by YNL.

Then sample fixes the bad attribute and re-issues the request, with
NLM_F_ECHO set. This flag causes the notification to be looped back
to the initiating socket (our socket). Sample parses this notification
to save the ifindex of the created netkit.

Sample then proceeds to list the devices. Line 8 above shows a dummy
device with two alt names. Lines 9 and 10 show the netkit devices
the sample itself created.

The "primary" and "policy" attrs are from inside the netkit submsg.
The string values are auto-generated for the enums by YNL.

To clean up sample deletes the interface it created (line 11).

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250515231650.1325372-10-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>