grant-table: type confusion in grant-copy

When grant-copy operations are processed, the respective grant may or may not already be in use by another operation (a mapping or another copy). For all copy operations the referenced guest frame is looked up. When another operation is already active for the grant (the grant is "pinned"), what is being supplied back to actually carry out permission checks and copy operation may not be consistent: The permission check may be carried out on a page different from the one involved in the copy.