loadgen: implement --data=  parameter
	pass file length through a parm global rather than as a formal.
loadgen - implement --data=
	clearly distinguish between bytes-to-read/write and file length

Signed-off-by: Mark Kampe <mark.kampe@gmail.com>
loadgen: update test cases to exploit --data=
	update random write/verify tests to do 64kb in a 16kb file

Signed-off-by: Mark Kampe <mark.kampe@gmail.com>