tools/testing/selftests: fix guard region test tmpfs assumption

tools/testing/selftests: fix guard region test tmpfs assumption

The current implementation of the guard region tests assume that /tmp is
mounted as tmpfs, that is shmem.

This isn't always the case, and at least one instance of a spurious test
failure has been reported as a result.

This assumption is unsafe, rushed and silly - and easily remedied by
simply using memfd, so do so.

We also have to fixup the readonly_file test to explicitly only be
applicable to file-backed cases.

Link: https://lkml.kernel.org/r/20250425162436.564002-1-lorenzo.stoakes@oracle.com
Fixes: 272f37d3e99a ("tools/selftests: expand all guard region tests to file-backed")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/linux-mm/a2d2766b-0ab4-437b-951a-8595a7506fe9@arm.com/
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>