netconsole: selftest: Delete all userdata keys

netconsole: selftest: Delete all userdata keys

Modify the cleanup function to remove all userdata keys created during the
test, instead of just deleting a single predefined key. This ensures a
more thorough cleanup of temporary resources.

Move the KEY_PATH variable definition inside the set_user_data function
to reduce global variables and improve encapsulation. The KEY_PATH
variable is now dynamically created when setting user data.

This change has no effect on the current test, while improving an
upcoming test that would create several userdata entries.

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