lib/radix-tree.c: fix uninitialized variable compilation warning

We need to set an initial value for offset to eliminate compilation
warning.

How to reproduce warning:

$ make -C tools/testing/radix-tree
radix-tree.c: In function `radix_tree_tag_clear':
radix-tree.c:1046:17: warning: `offset' may be used uninitialized in this function [-Wmaybe-uninitialized]
 1046 |                 node_tag_clear(root, parent, tag, offset);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Rong Tao <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Rtoax

lib/radix-tree.c

@@ -1029,7 +1029,7 @@ void *radix_tree_tag_clear(struct radix_tree_root *root,
 {
 	struct radix_tree_node *node, *parent;
 	unsigned long maxindex;
-	int offset;
+	int offset = 0;
 
 	radix_tree_load_root(root, &node, &maxindex);
 	if (index > maxindex)