NFS: restore module put when manager exits.

Commit f49169c9 ("NFSD: Remove svc_serv_ops::svo_module") removed
calls to module_put_and_kthread_exit() from threads that acted as SUNRPC
servers and had a related svc_serv_ops structure.  This was correct.

It ALSO removed the module_put_and_kthread_exit() call from
nfs4_run_state_manager() which is NOT a SUNRPC service.

Consequently every time the NFSv4 state manager runs the module count
increments and won't be decremented.  So the nfsv4 module cannot be
unloaded.

So restore the module_put_and_kthread_exit() call.

Fixes: f49169c9 ("NFSD: Remove svc_serv_ops::svo_module")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>