xen-swiotlb: When doing coherent alloc/dealloc check before swizzling the MFNs.

xen-swiotlb: When doing coherent alloc/dealloc check before swizzling the MFNs.

The process to swizzle a Machine Frame Number (MFN) is not always
necessary. Especially if we know that we actually do not have to do it.
In this patch we check the MFN against the device's coherent
DMA mask and if the requested page(s) are contingous. If it all checks
out we will just return the bus addr without doing the memory
swizzle.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 6810df88dcfc22de267caf23eb072ffb97b3c411)