Xen/gntdev: don't needlessly use kvcalloc()

Requesting zeroed memory when all of it will be overwritten subsequently
by all ones is a waste of processing bandwidth. In fact, rather than
recording zeroed ->grants[], fill that array too with more appropriate
"invalid" indicators.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/9a726be2-4893-8ffe-0ef1-b70dd1c229b1@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>