[IPV6]: Consolidate the ip cork destruction in ip6_output.c

The ip6_push_pending_frames and ip6_flush_pending_frames do the
same things to flush the sock's cork. Move this into a separate
function and save ~100 bytes from the .text

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