Changes Spray offset

Make data point at the beginning of the block.  A small allocation, it looks like:

	0:016> db 0x06333c50
	06333c50  41 41 41 41 42 42 42 42-43 43 43 43 44 44 44 44  AAAABBBBCCCCDDDD
	06333c60  45 45 45 45 46 46 46 46-47 47 47 47 cc cc cc cc  EEEEFFFFGGGG....
	06333c70  cc cc cc cc cc cc cc cc-cc cc cc cc 20 20 20 20  ............   

A larger block may have a header, followed by the data.