[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>