udpv6: Add lockless sendmsg() support

udpv6: Add lockless sendmsg() support

This commit adds the same functionaliy to IPv6 that
commit 903ab86d195cca295379699299c5fc10beba31c7
Author: Herbert Xu <[email protected]>
Date:   Tue Mar 1 02:36:48 2011 +0000

    udp: Add lockless transmit path

added to IPv4.

UDP transmit path can now run without a socket lock,
thus allowing multiple threads to send to a single socket
more efficiently.
This is only used when corking/MSG_MORE is not used.

Signed-off-by: Vladislav Yasevich <[email protected]>
Signed-off-by: David S. Miller <[email protected]>