radix tree test suite: fix compilation issue

radix tree test suite: fix compilation issue

Pulled from a patch from Matthew Wilcox entitled "xarray: Add definition
of struct xarray":

> From: Matthew Wilcox <mawilcox@microsoft.com>
> Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

  https://patchwork.kernel.org/patch/10341249/

These defines fix this compilation error:

  In file included from ./linux/radix-tree.h:6:0,
                   from ./linux/../../../../include/linux/idr.h:15,
                   from ./linux/idr.h:1,
                   from idr.c:4:
  ./linux/../../../../include/linux/idr.h: In function `idr_init_base':
  ./linux/../../../../include/linux/radix-tree.h:129:2: warning: implicit declaration of function `spin_lock_init'; did you mean `spinlock_t'? [-Wimplicit-function-declaration]
    spin_lock_init(&(root)->xa_lock);    \
    ^
  ./linux/../../../../include/linux/idr.h:126:2: note: in expansion of macro `INIT_RADIX_TREE'
    INIT_RADIX_TREE(&idr->idr_rt, IDR_RT_MARKER);
    ^~~~~~~~~~~~~~~

by providing a spin_lock_init() wrapper for the v4.17-rc* version of the
radix tree test suite.

Link: http://lkml.kernel.org/r/20180503192430.7582-3-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: CR, Sapthagirish <sapthagirish.cr@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>