IPv6: datagram_send_ctl() should exit immediately when an error occured

When an error occured, datagram_send_ctl() should exit immediately rather than
continue to run the for loop. Otherwise, the variable err might be changed and
the error might be hidden.

Fix this bug by using "goto" instead of "break".

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>