change find_ext_ns_id() to do collect_mntinfo(ext_ns) on demand

Currently we rely on the fact that ->mntinfo_list was already
collected by walk_namespaces(walk_all => true), but we are going
to change this.

This patch simply adds collect_mntinfo(ns) into find_ext_ns_id() if
->mntinfo_list == NULL. This is all we need for this ns_id if it was
not initialized by collect_mnt_namespaces().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -699,6 +699,9 @@ static struct ns_id *find_ext_ns_id(void)
 
 	for (ns = ns_ids; ns->next; ns = ns->next)
 		if (ns->pid == pid && ns->nd == &mnt_ns_desc) {
+			if (!ns->mnt.mntinfo_list &&
+			    !collect_mntinfo(ns, true))
+				break;
 			return ns;
 		}