mutable/servermap.py: oops, query N+e servers in MODE_WRITE, not k+e

under normal conditions, this wouldn't cause any problems, but if the shares
are really sparse (perhaps because new servers were added), then
file-modifies might stop looking too early and leave old shares in place

src/allmydata/mutable/servermap.py

@@ -434,7 +434,7 @@ class ServermapUpdater:
         if k is None:
             # make a guess
             k = 3
-        N = self._node.get_required_shares()
+        N = self._node.get_total_shares()
         if N is None:
             N = 10
         self.EPSILON = k