[DLCI]: Fix tiny race between module unload and sock_ioctl.

This is a narrow pedantry :) but the dlci_ioctl_hook check and call
should not be parted with the mutex lock.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>