vfs: fix io on levels

allow modify levels. IO to levels shold go
through MDS

Acked-by: Gerd Behrmann
Acked-by: Dmitry Litvintsev
Target: master, 0.7
(cherry picked from commit 93e8059d772c34a6815fc2367335b8589c80330b)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>