radix tree test suite: Support kmem_cache alignment

The radix tree doesn't use alignment, so the argument was ignored.
The maple tree needs its nodes to be aligned, so we need to pay attention
to the alignment argument.  Also change the types of 'size' and 'align'
to unsigned int to match commit f4957d5b.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>