netconsole: selftest: verify userdata entry limit

netconsole: selftest: verify userdata entry limit

Add a new selftest for netconsole that tests the userdata entry limit
functionality. The test performs two key verifications:

1. Create MAX_USERDATA_ITEMS (16) userdata entries successfully
2. Confirm that attempting to create an additional userdata entry fails

The selftest script uses the netcons library and checks the behavior
by attempting to create entries beyond the maximum allowed limit.

Signed-off-by: Breno Leitao <leitao@debian.org>
Tested-by: Simon Horman <horms@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250108-netcons_overflow_test-v3-4-3d85eb091bec@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>