vfs: spread struct mount - get_dominating_id / do_make_slave

next pile of horrors, similar to mnt_parent one; this time it's
mnt_master.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/namespace.c b/fs/namespace.c
index 9ceb03f..65d011f 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1053,7 +1053,7 @@
 		seq_printf(m, " shared:%i", mnt->mnt_group_id);
 	if (IS_MNT_SLAVE(mnt)) {
 		int master = mnt->mnt_master->mnt_group_id;
-		int dom = get_dominating_id(mnt, &p->root);
+		int dom = get_dominating_id(r, &p->root);
 		seq_printf(m, " master:%i", master);
 		if (dom && dom != master)
 			seq_printf(m, " propagate_from:%i", dom);