mm/gup.c: fix the comment of return value for populate_vma_page_range()

mm/gup.c: fix the comment of return value for populate_vma_page_range()

The return value of populate_vma_page_range() is consistent with
__get_user_pages(), and so is the function comment of return value.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: http://lkml.kernel.org/r/20200720034303.29920-1-tangyizhou@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>