selftests/mm: fix incorrect mmap() error handling with NULL instead of MAP_FAILED

selftests/mm: fix incorrect mmap() error handling with NULL instead of MAP_FAILED

mmap() returns MAP_FAILED, which is defined as (void *)-1, on error, not
NULL.  Several selftests incorrectly check the return value of mmap()
using !ptr or ptr == NULL, which would erroneously treat MAP_FAILED as a
valid pointer since MAP_FAILED is non-zero and non-NULL.  This can lead to
segfaults when mmap() actually fails under memory pressure.

Link: https://lore.kernel.org/20260513025223.592766-1-lihongfu@kylinos.cn
Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>