selftests/mm: use PIDFD_SELF in guard pages test

Now we have PIDFD_SELF available for process_madvise(), make use of it in
the guard pages test.

This is both more convenient and asserts that PIDFD_SELF works as expected.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/r/69fbbe088d3424de9983e145228459cb05a8f13d.1738268370.git.lorenzo.stoakes@oracle.com


Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Christian Brauner <brauner@kernel.org>