NFSD: replace delayed_work with work_struct for nfsd_client_shrinker

[ Upstream commit 7c24fa22

 ]

Since nfsd4_state_shrinker_count always calls mod_delayed_work with
0 delay, we can replace delayed_work with work_struct to save some
space and overhead.

Also add the call to cancel_work after unregister the shrinker
in nfs4_state_shutdown_net.

Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>