net/socket.c : introduce helper function do_sock_sendmsg to replace reduplicate code

Introduce helper function do_sock_sendmsg() to simplify sock_sendmsg{_nosec},
and replace reduplicate code.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>