xen/blkback: simplify free_persistent_gnts() interface

The interface of free_persistent_gnts() can be simplified, as there is
only a single caller of free_persistent_gnts() and the 2nd and 3rd
parameters are easily obtainable via the ring pointer, which is passed
as the first parameter anyway.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>