vma_merge: set vma iterator to correct position.

When merging the previous value, set the vma iterator to the previous
slot.  Don't use the vma iterator to get the next/prev so that it is in
the correct position for a write.

Link: https://lkml.kernel.org/r/20230120162650.984577-50-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>