NFSD: Capture every CB state transition

We were missing one.

As a clean-up, add a helper that sets the new CB state and fires
a tracepoint. The tracepoint fires only when the state changes, to
help reduce trace log noise.

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