ceph: use strong hash function for mapping objects to pgs

We were using the (weak) dcache hash function, but it was leaving lower
bits consecutive for consecutive (inode) objects.  We really want to make
the object to pg mapping random and uniform, so use a proper hash function
here.

This is Robert Jenkin's public domain hash function (with some minor
cleanup):
	http://burtleburtle.net/bob/hash/evahash.html



This is a protocol revision.

Signed-off-by: Sage Weil <sage@newdream.net>