mounts: Make sure all namespaces has the same root source

Currently we support nested mount namespaces, only if all of them have
the same path to the root.

crtools doesn't mount root, so it should be bind-mounted for all namespaces.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -233,6 +233,12 @@ static struct mount_info *mnt_build_ids_tree(struct mount_info *list)
 					m->mnt_id, m->parent_mnt_id, m->mountpoint,
 					root ? "found" : "not found");
 			if (root && m->is_ns_root) {
+				if (!mounts_equal(root, m, true) ||
+						strcmp(root->root, m->root)) {
+					pr_err("Nested mount namespaces with different roots are not supported yet");
+					return NULL;
+				}
+
 				/*
 				 * A root of a sub mount namespace is
 				 * mounted in a temporary directory in the