[IPV6]: Fix again the fl6_sock_lookup() fixed locking

YOSHIFUJI fairly pointed out, that the users increment should
be done under the ip6_sk_fl_lock not to give IPV6_FL_A_PUT a
chance to put this count to zero and release the flowlabel.

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