swiotlb: Introduce swiotlb_max_mapping_size()

swiotlb: Introduce swiotlb_max_mapping_size()

BugLink: https://bugs.launchpad.net/bugs/1821607
commit abe420bfae528c92bd8cc5ecb62dc95672b1fd6f upstream.

The function returns the maximum size that can be remapped
by the SWIOTLB implementation. This function will be later
exposed to users through the DMA-API.

Cc: stable@vger.kernel.org
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>