NFSD: Remove BUG_ON in nfsd4_process_cb_update()

Don't kill the kworker thread, and don't panic while cl_lock is
held. There's no need for scorching the earth here.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>