nfs4:  shutdown state handler on destroy

for graceful dcache shutdown

Target: master, 2.10
Acked-By: Paul Millar
Require-book: no
Require-notes: no
(cherry picked from commit 55eb29f3f3038325f60e187d34c20546580f6c03)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>