gnttab: cleanup of number-of-active-frames calculations

gnttab: cleanup of number-of-active-frames calculations

max_nr_active_grant_frames() is merly is special case of
num_act_frames_from_sha_frames(), so there's no need to have a special
case implementation for it.

Further, some of the related definitions (including the "struct
active_grant_entry" definition itself) can (and hence should) really be
private to grant_table.c.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>