swiotlb-xen: static inline xen_phys_to_bus, xen_bus_to_phys, xen_virt_to_bus...

swiotlb-xen: static inline xen_phys_to_bus, xen_bus_to_phys, xen_virt_to_bus and range_straddles_page_boundary

This functions are small and called frequently. Static inline them.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>