block: simplify bdrv_find_base() and bdrv_find_overlay()

This simplifies the function bdrv_find_overlay().  With this change,
bdrv_find_base() is just a subset of usage of bdrv_find_overlay(),
so this also takes advantage of that.
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -2508,32 +2508,23 @@ int bdrv_change_backing_file(BlockDriverState *bs,
  *
  * Returns NULL if bs is not found in active's image chain,
  * or if active == bs.
+ *
+ * Returns the bottommost base image if bs == NULL.
  */
 BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
                                     BlockDriverState *bs)
 {
-    BlockDriverState *overlay = NULL;
-    BlockDriverState *intermediate;
-
-    assert(active != NULL);
-    assert(bs != NULL);
-
-    /* if bs is the same as active, then by definition it has no overlay
-     */
-    if (active == bs) {
-        return NULL;
+    while (active && bs != active->backing_hd) {
+        active = active->backing_hd;
     }
 
-    intermediate = active;
-    while (intermediate->backing_hd) {
-        if (intermediate->backing_hd == bs) {
-            overlay = intermediate;
-            break;
-        }
-        intermediate = intermediate->backing_hd;
-    }
+    return active;
+}
 
-    return overlay;
+/* Given a BDS, searches for the base layer. */
+BlockDriverState *bdrv_find_base(BlockDriverState *bs)
+{
+    return bdrv_find_overlay(bs, NULL);
 }
 
 typedef struct BlkIntermediateStates {
@@ -4326,22 +4317,6 @@ int bdrv_get_backing_file_depth(BlockDriverState *bs)
     return 1 + bdrv_get_backing_file_depth(bs->backing_hd);
 }
 
-BlockDriverState *bdrv_find_base(BlockDriverState *bs)
-{
-    BlockDriverState *curr_bs = NULL;
-
-    if (!bs) {
-        return NULL;
-    }
-
-    curr_bs = bs;
-
-    while (curr_bs->backing_hd) {
-        curr_bs = curr_bs->backing_hd;
-    }
-    return curr_bs;
-}
-
 /**************************************************************/
 /* async I/Os */