fix xen_swiotlb_dma_mmap prototype

xen_swiotlb_dma_mmap was backported from v4.10, but older
kernels before commit 00085f1e ("dma-mapping: use unsigned long
for dma_attrs") use a different signature:

arm/xen/mm.c:202:10: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
  .mmap = xen_swiotlb_dma_mmap,
          ^~~~~~~~~~~~~~~~~~~~
arm/xen/mm.c:202:10: note: (near initialization for 'xen_swiotlb_dma_ops.mmap')

This adapts the patch to the old calling conventions.

Fixes: "swiotlb-xen: implement xen_swiotlb_dma_mmap callback"
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>