xen/blkback: Fix interface.c checkpatch warnings .. except

+               sring_x86_64 = (struct blkif_x86_64_sring *)blkif->blk_ring_area->addr;

WARNING: line over 80 characters
+               BACK_RING_INIT(&blkif->blk_rings.x86_64, sring_x86_64, PAGE_SIZE);

as breaking them up really does not help that much.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>