Currently the pthread allocation for each array item is based on the size
of a pthread_t pointer and should be the size of the pthread_t structure,
so the allocation is under-allocating the correct size.  Fix this by using
the size of each element in the pthreads array.

Static analysis cppcheck reported:
tools/testing/radix-tree/regression1.c:180:2: warning: Size of pointer
'threads' used instead of size of its data. [pointerSize]

Link: https://lkml.kernel.org/r/20230727160930.632674-1-colin.i.king@gmail.com
Fixes: 1366c37e ("radix tree test harness")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>