gnttab: eliminate several explicit version checks

gnttab: eliminate several explicit version checks

By having nr_grant_entries() return zero when the grant table version
is still unset we can reduce the number of error paths and at once fix
grant_map_exists() running into the being removed ASSERT() when called
for a page owned by a domain not having its grant table set up yet.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>