xen/blkback: remove unused pers_gnts_lock from struct xen_blkif_ring

pers_gnts_lock isn't being used anywhere. Remove it.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>