selftests/vm/keys: fix a broken reference at protection_keys.c

Changeset 1eecbcdca2bd ("docs: move protection-keys.rst to the core-api book")
from Jun 7, 2019 converted protection-keys.txt file to ReST.

A recent change at protection_keys.c partially reverted such
changeset, causing it to point to a non-existing file:

- * Tests x86 Memory Protection Keys (see Documentation/core-api/protection-keys.rst)
+ * Tests Memory Protection Keys (see Documentation/vm/protection-keys.txt)

It sounds to me that the changeset that introduced such change
4645e3563673 ("selftests/vm/pkeys: rename all references to pkru to a generic name")
could also have other side effects, as it sounds that it was not
generated against uptream code, but, instead, against a version
older than Jun 7, 2019.

Fixes: 4645e3563673 ("selftests/vm/pkeys: rename all references to pkru to a generic name")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/cf65aa052669f55b9dc976a5c8026aef5840741d.1592895969.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>