dmaengine: mv_xor: Fix incorrect offset in dma_map_page()

Upon booting, I occasionally spotted some BUGs triggered by the internal
DMA test routine executed upon driver probing. This was detected by
SLUB_DEBUG ("Freechain corrupt" or "Redzone overwritten"). Tracking
this down located a problem in passing 0 as offset in dma_map_page().
As kmalloc, especially when used with SLUB_DEBUG, may return a non page
aligned address.

This patch fixes this issue by passing the correct offset in
dma_map_page().

Tested on a custom Armada XP board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>