Commit 
a46b53672b2c2e3770b38a4abf90d16364d2584b ("xen/blkfront: cleanup
stale persistent grants") introduced a regression as purged persistent
grants were not pu into the list of free grants again. Correct that.
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
                        list_del(&gnt_list_entry->node);
                        gnttab_end_foreign_access(gnt_list_entry->gref, 0, 0UL);
                        rinfo->persistent_gnts_c--;
-                       __free_page(gnt_list_entry->page);
-                       kfree(gnt_list_entry);
+                       gnt_list_entry->gref = GRANT_INVALID_REF;
+                       list_add_tail(&gnt_list_entry->node, &rinfo->grants);
                }
 
                spin_unlock_irqrestore(&rinfo->ring_lock, flags);