rbd: fix leak of format 2 snapshot context

When rbd_dev_v2_refresh() is called, the rbd device already has a
snapshot context associated with it.  But that never gets freed,
the pointer just gets overwritten.

Fix this by dropping the rbd device's reference to the snapshot
context before overwriting the pointer.

Because ceph_put_snap_context() already handles for a null pointer
we don't need to check for that (for the probe case, where no
context has yet been assigned).

This resolves:
    http://tracker.ceph.com/issues/4912Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -4004,6 +4004,7 @@ static int rbd_dev_v2_snap_context(struct rbd_device *rbd_dev)
 	for (i = 0; i < snap_count; i++)
 		snapc->snaps[i] = ceph_decode_64(&p);
 
+	ceph_put_snap_context(rbd_dev->header.snapc);
 	rbd_dev->header.snapc = snapc;
 
 	dout("  snap context seq = %llu, snap_count = %u\n",