Add zpool_disable_datasets_os() / zfs_unmount_os()

zpool_disable_datasets_os():
macOS needs to do a bunch of work to kick everything off zvols.

zfs_unmount_os():
This allows us to unmount any zvols that may be mounted. Like with
zfs destroy foo/vol

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12436