mm/migrate.c: stop using 0 as NULL pointer

mm/migrate.c:1198:24: warning: Using plain integer as NULL pointer

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3080
Link: https://lkml.kernel.org/r/20221116012345.84870-1-yang.lee@linux.alibaba.com


Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>