selftests/mm/cow: fix the incorrect error handling

Error handling doesn't check the correct return value.  This patch will
fix it.

Link: https://lkml.kernel.org/r/20250312043840.71799-1-cyan.yang@sifive.com
Fixes: f4b5fd6 ("selftests/vm: anon_cow: THP tests")
Signed-off-by: Cyan Yang <cyan.yang@sifive.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: cyanyang-sifive

tools/testing/selftests/mm/cow.c

@@ -876,7 +876,7 @@ static void do_run_with_thp(test_fn fn, enum thp_run thp_run, size_t thpsize)
 		mremap_size = thpsize / 2;
 		mremap_mem = mmap(NULL, mremap_size, PROT_NONE,
 				  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-		if (mem == MAP_FAILED) {
+		if (mremap_mem == MAP_FAILED) {
 			ksft_test_result_fail("mmap() failed\n");
 			goto munmap;
 		}