Revert "xen/blkfront: When purging persistent grants, keep them in the buffer"

Fix didn't work for all cases, reverting to add a (hopefully)
better fix.

This reverts commit f151ba98.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkfront.c

@@ -2667,9 +2667,11 @@ static void purge_persistent_grants(struct blkfront_info *info)
 			    gnttab_query_foreign_access(gnt_list_entry->gref))
 				continue;
 
+			list_del(&gnt_list_entry->node);
 			gnttab_end_foreign_access(gnt_list_entry->gref, 0, 0UL);
-			gnt_list_entry->gref = GRANT_INVALID_REF;
 			rinfo->persistent_gnts_c--;
+			__free_page(gnt_list_entry->page);
+			kfree(gnt_list_entry);
 		}
 
 		spin_unlock_irqrestore(&rinfo->ring_lock, flags);