[AF_UNIX]: Remove superfluous reference counting in unix_stream_sendmsg

AF_UNIX stream socket performance on P4 CPUs tends to suffer due to a
lot of pipeline flushes from atomic operations.  The patch below
removes the sock_hold() and sock_put() in unix_stream_sendmsg().  This
should be safe as the socket still holds a reference to its peer which
is only released after the file descriptor's final user invokes
unix_release_sock().  The only consideration is that we must add a
memory barrier before setting the peer initially.

Signed-off-by: Benjamin LaHaise <benjamin.c.lahaise@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>