selftests/mm/cow: fix the incorrect error handling

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: f4b5fd6946e2 ("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>