xen/swiotlb: fix allocated size

The allocated size in xen_swiotlb_alloc_coherent() and
xen_swiotlb_free_coherent() is calculated wrong for the case of
XEN_PAGE_SIZE not matching PAGE_SIZE. Fix that.

Fixes: 7250f422 ("xen-swiotlb: use actually allocated size on check physical continuous")
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>