1: add another compiler barrier to acquire_grant_for_copy()
2: limit scope of transitive-only variables in acquire_grant_for_copy()
3: fold redundant error code paths in acquire_grant_for_copy()
4: use available local var in map_grant_ref()
5: drop local "ld" from unmap_common_complete()
6: rename a parameter of gnttab_populate_status_frames()
7: unreachable code when GNTTAB_MAX_VERSION < 2

Jan