nfsv4: add state dispose listener

this mechanism is used by proxy-io module to
free allocated resources.

Acked-by: Albert Rossi
Target: master, 0.6
(cherry picked from commit 5958f9730d73771ce14dde224d2918f67ee98b35)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>