xen: gntdev: move use of GNTMAP_contains_pte next to the map_op

This flag controls the meaning of gnttab_map_grant_ref.host_addr and
specifies that the field contains a reference to the pte entry to be
used to perform the mapping. Therefore move the use of this flag to
the point at which we actually use a reference to the pte instead of
something else, splitting up the usage of the flag in this way is
confusing and potentially error prone.

The other flags are all properties of the mapping itself as opposed to
properties of the hypercall arguments and therefore it make sense to
continue to pass them round in map->flags.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>
Cc: Derek G. Murray <Derek.Murray@cl.cam.ac.uk>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>