radix tree test suite: fix allocation calculation in kmem_cache_alloc_bulk()

The bulk allocation is iterating through an array and storing enough
memory for the entire bulk allocation instead of a single array entry. 
Only allocate an array element of the size set in the kmem_cache.

Link: https://lkml.kernel.org/r/20230929201359.2857583-1-Liam.Howlett@oracle.com
Fixes: cc86e0c2

 ("radix tree test suite: add support for slab bulk APIs")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>