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

lib/libzfs/os/linux/libzfs_mount_os.c

@@ -411,3 +411,15 @@ zfs_mount_delegation_check(void)
 {
 	return ((geteuid() != 0) ? EACCES : 0);
 }
+
+/* Called from the tail end of zpool_disable_datasets() */
+void
+zpool_disable_datasets_os(zpool_handle_t *zhp, boolean_t force)
+{
+}
+
+/* Called from the tail end of zfs_unmount() */
+void
+zpool_disable_volume_os(const char *name)
+{
+}