gnttab: adjust unmap checking of dev_bus_addr

gnttab: adjust unmap checking of dev_bus_addr

There's no point checking ->dev_bus_addr when GNTMAP_device_map isn't
set (and hence the field isn't going to be consumed). And if there is a
mismatch, use the so far unused GNTST_bad_dev_addr error indicator - if
not here, where else would this (so far unused) value be used?

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>