makefs: zfs dataset mountpoints are considered non-empty

The dataset mountpoints get link count and size calculated by
content of dataset root directory, but this will break libzfs
dir_is_empty_stat() test.

For fix, we need to check if the directory is mountpoint for
dataset, and calculate the link count and size accordingly.

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D51593