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,
                                 ^

Closes zfsonlinux/zfs#1480

Signed-off-by: Richard Yao <ryao@gentoo.org>