cid_recov_dir needs to be there when there is a ref on it

nfs4_record_revoke() uses cid_recov_dir. Currently cid_recov_dir is
freed when clientis gets expired. There is no way to stop the clientid
getting expired other than holding cr_mutex which will have ABBA
deadlock issues with state_lock in nfs4_record_revoke. If this truly
needs to be freed when a client gets expired, we should use cid_mutex.
For now, freeing this buffer when the refcount goes to zero.

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>