selftests/mm: two fixes for the pfnmap test

selftests/mm: two fixes for the pfnmap test

When unregistering the signal handler, we have to pass SIG_DFL, and
blindly reading from PFN 0 and PFN 1 seems to be problematic on !x86
systems.  In particularly, on arm64 tx2 machines where noting resides at
these physical memory locations, we can generate RAS errors.

Let's fix it by scanning /proc/iomem for actual "System RAM".

Link: https://lkml.kernel.org/r/20250528195244.1182810-1-david@redhat.com
Fixes: 2616b370323a ("selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/all/232960c2-81db-47ca-a337-38c4bce5f997@arm.com/T/#u
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Tested-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>