NFSD: Adjust cb_shutdown tracepoint

[ Upstream commit b200f0e3 ]

Show when the upper layer requested a shutdown. RPC tracepoints can
already show when rpc_shutdown_client() is called.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>