mm: use vma_pages() to replace (vm_end - vm_start) >> PAGE_SHIFT

(*->vm_end - *->vm_start) >> PAGE_SHIFT operation is implemented
as a inline funcion vma_pages() in linux/mm.h, so using it.

Signed-off-by: Libin <huawei.libin@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

diff --git a/mm/mmap.c b/mm/mmap.c
index f681e18..8468ffd 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -955,7 +955,7 @@
 	if (is_mergeable_vma(vma, file, vm_flags) &&
 	    is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma)) {
 		pgoff_t vm_pglen;
-		vm_pglen = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+		vm_pglen = vma_pages(vma);
 		if (vma->vm_pgoff + vm_pglen == vm_pgoff)
 			return 1;
 	}