Btrfs: optimize to remove unnecessary removal with ulist reallocation

Here we are not going to free memory, no need to remove every node
one by one, just init root node here is ok.

Cc:  Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: NWang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/ulist.c

@@ -207,9 +207,6 @@ int ulist_add_merge(struct ulist *ulist, u64 val, u64 aux,
 		void *old = NULL;
 		int i;
 
-		for (i = 0; i < ulist->nnodes; i++)
-			rb_erase(&ulist->nodes[i].rb_node, &ulist->root);
-
 		/*
 		 * if nodes_alloced == ULIST_SIZE no memory has been allocated
 		 * yet, so pass NULL to krealloc
@@ -234,6 +231,7 @@ int ulist_add_merge(struct ulist *ulist, u64 val, u64 aux,
 		 * pointers, so we have to do it ourselves.  Otherwise we may
 		 * be bitten by crashes.
 		 */
+		ulist->root = RB_ROOT;
 		for (i = 0; i < ulist->nnodes; i++) {
 			ret = ulist_rbtree_insert(ulist, &ulist->nodes[i]);
 			if (ret < 0)