drivers/xen/gntdev: use xen_pv_domain() instead of cached value

Eliminate the use_ptemod variable by replacing its use cases with
xen_pv_domain().

Instead of passing the xen_pv_domain() return value to
gntdev_ioctl_dmabuf_exp_from_refs(), use xen_pv_domain() in that
function.

Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250826145608.10352-4-jgross@suse.com>