test suite: use %zu to print size_t

On 32-bit, we can't use %lu to print a size_t variable and gcc warns us
about it.  Shame it doesn't warn about it on 64-bit.

Link: https://lkml.kernel.org/r/20250403003311.359917-1-Liam.Howlett@oracle.com


Fixes: cc86e0c2 ("radix tree test suite: add support for slab bulk APIs")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>