xen/gntalloc: fix oops after runnning out of grant refs

Only set gref->gref_id if foreign access was successfully granted and
the grant ref is valid.

If gref->gref_id == -ENOSPC the test in __del_gref() would incorrectly
attempt to end foreign access (because grant_ref_t is unsigned).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Dave Scott <dave.scott@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>