dup_mnt_ns(): get rid of pointless grabbing of vfsmount_lock

mnt_list is protected by namespace_sem, not vfsmount_lock
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -2444,9 +2444,7 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
 		return ERR_CAST(new);
 	}
 	new_ns->root = new;
-	br_write_lock(&vfsmount_lock);
 	list_add_tail(&new_ns->list, &new->mnt_list);
-	br_write_unlock(&vfsmount_lock);
 
 	/*
 	 * Second pass: switch the tsk->fs->* elements and mark new vfsmounts