radix tree test suite: fix incorrect allocation size for pthreads

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/[e-mail]
Fixes: 1366c37ed84b ("radix tree test harness")
Signed-off-by: Colin Ian King <[e-mail]>
Cc: Konstantin Khlebnikov <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/radix-tree/regression1.c

···
 	nr_threads = 2;
 	pthread_barrier_init(&worker_barrier, NULL, nr_threads);
 
-	threads = malloc(nr_threads * sizeof(pthread_t *));
+	threads = malloc(nr_threads * sizeof(*threads));
 
 	for (i = 0; i < nr_threads; i++) {
 		arg = i;