NTB: transport: Try harder to alloc an aligned MW buffer

Be a little wasteful if the (likely CMA) message window buffer is not
suitably aligned after our first attempt; allocate a buffer twice as big
as we need and manually align our MW buffer within it.

This was needed on Intel Broadwell DE platforms with intel_iommu=off

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>