nfs4: reuse existing mover if client comes with the same state id

if we already have returned a mover the client, but client asks for it again.
Currently, we will create a new mover, but client will use ( and close ) only
one of them.

Kill existing mover if we return error back to client.

Acked-by: Gerd Behrmann
Target: master
Require-notes: no
Require-book: no