[AF_UNIX]: scm: better initialization

Instead of doing a memset then initialization of the fields of the scm
structure, just initialize all the members explicitly.  Prevent reloading
of current on x86 and x86-64 by storing the value in a local variable for
subsequent dereferences.  This is worth a ~7KB/s increase in af_unix
bandwidth.  Note that we avoid the issues surrounding potentially
uninitialized members of the ucred structure by constructing a struct
ucred instead of assigning the members individually, which forces the
compiler to zero any padding.

[ I modified the patch not to use the aggregate assignment since
  gcc-3.4.x and earlier cannot optimize that properly at all even
  though gcc-4.0.x and later can -DaveM ]

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

include/net/scm.h

@@ -37,10 +37,12 @@ static __inline__ void scm_destroy(struct scm_cookie *scm)
 static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
 			       struct scm_cookie *scm)
 {
-	memset(scm, 0, sizeof(*scm));
-	scm->creds.uid = current->uid;
-	scm->creds.gid = current->gid;
-	scm->creds.pid = current->tgid;
+	struct task_struct *p = current;
+	scm->creds.uid = p->uid;
+	scm->creds.gid = p->gid;
+	scm->creds.pid = p->tgid;
+	scm->fp = NULL;
+	scm->seq = 0;
 	if (msg->msg_controllen <= 0)
 		return 0;
 	return __scm_send(sock, msg, scm);