selftests/mm: protection_keys: fix dead code

The while loop doesn't execute and following warning gets generated:

protection_keys.c:561:15: warning: code will never be executed
[-Wunreachable-code]
                int rpkey = alloc_random_pkey();

Let's enable the while loop such that it gets executed nr_iterations
times. Simplify the code a bit as well.

Link: https://lkml.kernel.org/r/20250912123025.1271051-3-usama.anjum@collabora.com


Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>