review request (S) - 6861660 OopMapBlock count/size confusion

[John Coomes]

Several places assumed OopMapBlock was one HeapWord in size, but that
has to change.

	http://cr.openjdk.java.net/~jcoomes/6861660-oopmapblock/

Comments appreciated.  This is a prerequisite to fixing

	6845368 large objects cause a crash or unexpected exception

-John