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

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/tencent_DF74099967595DCEA93CBDC28D062026180A@qq.com
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>