mm/mmap/vma_merge: initialize mid and next in natural order

It is more intuitive to go from prev to mid and then next.  No functional
change.

Link: https://lkml.kernel.org/r/20230309111258.24079-6-vbabka@suse.czSigned-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -912,10 +912,11 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	if (vm_flags & VM_SPECIAL)
 		return NULL;
 
-	next = find_vma(mm, prev ? prev->vm_end : 0);
-	mid = next;
-	if (next && next->vm_end == end)		/* cases 6, 7, 8 */
-		next = find_vma(mm, next->vm_end);
+	mid = find_vma(mm, prev ? prev->vm_end : 0);
+	if (mid && mid->vm_end == end)			/* cases 6, 7, 8 */
+		next = find_vma(mm, mid->vm_end);
+	else
+		next = mid;
 
 	/* verify some invariant that must be enforced by the caller */
 	VM_WARN_ON(prev && addr <= prev->vm_start);