Use MAXPATHLEN instead of sizeof in snprintf

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 <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1480