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>

drivers/block/xen-blkback/common.h

@@ -269,7 +269,6 @@ struct xen_blkif_ring {
 	wait_queue_head_t	pending_free_wq;
 
 	/* Tree to store persistent grants. */
-	spinlock_t		pers_gnts_lock;
 	struct rb_root		persistent_gnts;
 	unsigned int		persistent_gnt_c;
 	atomic_t		persistent_gnt_in_use;