[IPV6]: Lost locking when inserting a flowlabel in ipv6_fl_list

The new flowlabels should be inserted into the sock list
under the ip6_sk_fl_lock. This was lost in one place.

This list is naturally protected with the socket lock, but
the fl6_sock_lookup() is called without it, so another
protection is required.

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