mm/mremap: fix dup_anon_vma() in vma_merge() case 4

mm/mremap: fix dup_anon_vma() in vma_merge() case 4

In case 4, we are shrinking 'prev' (PPPP in the comment) and expanding
'mid' (NNNN).  So we need to make sure 'mid' clones the anon_vma from
'prev', if it doesn't have any.  After commit 0503ea8f5ba7 ("mm/mmap:
remove __vma_adjust()") we can fail to do that due to wrong parameters for
dup_anon_vma().  The call is a no-op because res == next, adjust == mid
and mid == next.  Fix it.

Link: https://lkml.kernel.org/r/ad91d62b-37eb-4b73-707a-3c45c9e16256@suse.cz
Fixes: 0503ea8f5ba7 ("mm/mmap: remove __vma_adjust()")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>