userfaultfd: use helper function range_in_vma()

userfaultfd: use helper function range_in_vma()

We can use range_in_vma() to check if dst_start, dst_start + len are
within the dst_vma range.  Minor readability improvement.

Link: https://lkml.kernel.org/r/20230417003919.930515-1-zhangpeng362@huawei.com
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>