selftests/mm: fix a "possibly uninitialized" warning in pkey-x86.h

This fixes a real bug, too, because xstate_size() was assuming that
the stack variable xstate_size was initialized to zero. That's not
guaranteed nor even especially likely.

Link: https://lkml.kernel.org/r/20230606071637.267103-8-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>