[IPV6]: Lost locking in fl6_sock_lookup

This routine scans the ipv6_fl_list whose update is
protected with the socket lock and the ip6_sk_fl_lock.

Since the socket lock is not taken in the lookup, use
the other one.

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