nfsd: unlock on error in manage_cpntf_state()

We are holding the "nn->s2s_cp_lock" so we can't return directly
without unlocking first.

Fixes: f3dee17721a0 ("NFSD check stateids against copy stateids")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>