Use MAXPATHLEN instead of sizeof in snprintf

This silences a GCC 4.8.0 warning by fixing a programming error
caught by static analysis:

../../cmd/ztest/ztest.c: In function ‘ztest_vdev_aux_add_remove’:
../../cmd/ztest/ztest.c:2584:33: error: argument to ‘sizeof’
  in ‘snprintf’ call is the same expression as the destination;
  did you mean to provide an explicit length?
  [-Werror=sizeof-pointer-memaccess]
  (void) snprintf(path, sizeof (path), ztest_aux_template,
                                 ^

Signed-off-by: Richard Yao <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #1480

cmd/ztest/ztest.c

@@ -2581,7 +2581,7 @@ ztest_vdev_aux_add_remove(ztest_ds_t *zd, uint64_t id)
 		zs->zs_vdev_aux = 0;
 		for (;;) {
 			int c;
-			(void) snprintf(path, sizeof (path), ztest_aux_template,
+			(void) snprintf(path, MAXPATHLEN, ztest_aux_template,
 			    ztest_opts.zo_dir, ztest_opts.zo_pool, aux,
 			    zs->zs_vdev_aux);
 			for (c = 0; c < sav->sav_count; c++)