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/libzfs_mount.c

@@ -672,6 +672,8 @@ zfs_unmount(zfs_handle_t *zhp, const char *mountpoint, int flags)
 		}
 	}
 
+	zpool_disable_volume_os(zhp->zfs_name);
+
 	return (0);
 }
 
@@ -1637,6 +1639,8 @@ zpool_disable_datasets(zpool_handle_t *zhp, boolean_t force)
 			remove_mountpoint(sets[i].dataset);
 	}
 
+	zpool_disable_datasets_os(zhp, force);
+
 	ret = 0;
 out:
 	(void) fclose(mnttab);