Ram disk is allocating 8x more space than required for diff data.
For large RAM disk test, there is small potential for memory
starvation.

(Use block_size when calculating total_sg_needed - sagi + nab)
Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: Quinn Tran <quinn.tran@qlogic.com>
Cc: <stable@vger.kernel.org> #3.14+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>