nfs_crhold
struct ucred *nfs_crhold(struct ucred *cred);
np->n_rucred = nfs_crhold(cnp->cn_cred);
np->n_wucred = nfs_crhold(cnp->cn_cred);
ncred = nfs_crhold(cred);
ncred = nfs_crhold(cred);
ncred = nfs_crhold(ap->a_cred);
ncred = nfs_crhold(ap->a_cred);