Anton noticed in some traces that we were spending an awfully long time
doing a memset.  The ppc64 memset is basically unchanged from the ppc32
version, and it only does 4-byte stores and doesn't unroll the loop. 
Here's a memset that performs a bit better.

I have been using it for 3 weeks now, and Anton has tested it on a
variety of machines, without problems. 
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>