Xen/gntdev: don't needlessly allocate k{,un}map_ops[]

They're needed only in the not-auto-translate (i.e. PV) case; there's no
point in allocating memory that's never going to get accessed.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/180d50cb-5531-8952-4bf0-d65c554638ed@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>