vfs_quotactl
if (vfsops->vfs_quotactl == NULL) {
vfsops->vfs_quotactl = vfs_stdquotactl;
error = (mp->mnt_op->vfs_quotactl)(mp, cmds, uid, arg, cred);
int vfs_quotactl(struct mount *mp, int cmds, uid_t uid, caddr_t arg,
vfs_quotactl_t *vfs_quotactl;
vfs_quotactl(MP, C, U, A, CRED)
.vfs_quotactl = ufs_quotactl,
.vfs_quotactl = nullfs_quotactl,
.vfs_quotactl = ufs_quotactl,