posix: Use PROT_READ for tst-mmap address probe

Use PROT_READ for the anonymous mmap used to obtain a page-aligned
address to avoid possible unrelated permission-related mmap
failures.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

posix/tst-mmap.c

@@ -93,7 +93,7 @@ do_test (void)
     }
 
   /* Get a valid, page-aligned address.  */
-  mem = mmap (NULL, 2 * ps, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  mem = mmap (NULL, 2 * ps, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
   if (mem != MAP_FAILED)
     {
       void *addr = (char *) mem + 1;