nfsd: find_cpntf_state cleanup

I think this unusual use of struct compound_state could cause confusion.

It's not that much more complicated just to open-code this stateid
lookup.

The only change in behavior should be a different error return in the
case the copy is using a source stateid that is a revoked delegation,
but I doubt that matters.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
[ cel: squashed in fix reported by Coverity ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>