selftests: revocable: Add kselftest cases

selftests: revocable: Add kselftest cases

Add kselftest cases for the revocable API.

The test consists of three parts:
- A kernel module (revocable_test.ko) that creates a debugfs interface
  with `/provider` and `/consumer` files.
- A user-space C program (revocable_test) that uses the kselftest
  harness to interact with the debugfs files.
- An orchestrating shell script (test-revocable.sh) that loads the
  module, runs the C program, and unloads the module.

The test cases cover the following scenarios:
- Basic: Verifies that a consumer can successfully access the resource
  provided via the provider.
- Revocation: Verifies that after the provider revokes the resource,
  the consumer correctly receives a NULL pointer on a subsequent access.
- Try Access Macro: Same as "Revocation" but uses the
  REVOCABLE_TRY_ACCESS_WITH() and REVOCABLE_TRY_ACCESS_SCOPED().

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://patch.msgid.link/20260116080235.350305-4-tzungbi@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>