Use hlists for the PID hashes.  This halves the memory footprint of these
hashes.  No benchmarks, but I think this is a worthy improvement because
the hashes are something that would be likely to have significant portions
loaded into the cache of every CPU on some workloads.

This comes at the "expense" of
	1. reintroducing the memory  prefetch into the hash traversal loop;
	2. adding new pids to the head of the list instead of the tail. I
	   suspect that if this was a big problem then the hash isn't sized
	   well or could benefit from moving hot entries to the head.

Also, account for all the pid hashes when reporting hash memory usage.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>