mm/mempolicy.c: fix pgoff in mbind vma merge

commit 9d8cebd4bcd7 ("mm: fix mbind vma merge problem") didn't really
fix the mbind vma merge problem due to wrong pgoff value passing to
vma_merge(), which made vma_merge() always return NULL.

Before the patch applied, we are getting a result like:

addr = 0x7fa58f00c000
[snip]
7fa58f00c000-7fa58f00d000 rw-p 00000000 00:00 0
7fa58f00d000-7fa58f00e000 rw-p 00000000 00:00 0
7fa58f00e000-7fa58f00f000 rw-p 00000000 00:00 0

here 7fa58f00c000->7fa58f00f000 we get 3 VMAs which are expected to be
merged described as described in commit 9d8cebd.

Re-testing the patched kernel with the reproducer provided in commit
9d8cebd, we get the correct result:

addr = 0x7ffa5aaa2000
[snip]
7ffa5aaa2000-7ffa5aaa6000 rw-p 00000000 00:00 0
7fffd556f000-7fffd5584000 rw-p 00000000 00:00 0 [stack]

Signed-off-by: Caspar Zhang <[e-mail]>
Cc: KOSAKI Motohiro <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Lee Schermerhorn <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/mempolicy.c

···
 	struct vm_area_struct *prev;
 	struct vm_area_struct *vma;
 	int err = 0;
-	pgoff_t pgoff;
 	unsigned long vmstart;
 	unsigned long vmend;
 
···
 		vmstart = max(start, vma->vm_start);
 		vmend   = min(end, vma->vm_end);
 
-		pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT);
 		prev = vma_merge(mm, prev, vmstart, vmend, vma->vm_flags,
-				  vma->anon_vma, vma->vm_file, pgoff, new_pol);
+				  vma->anon_vma, vma->vm_file, vma->vm_pgoff,
+				  new_pol);
 		if (prev) {
 			vma = prev;
 			next = vma->vm_next;