ffs_truncate
int ffs_truncate(struct vnode *, off_t, int, struct ucred *);
(void)ffs_truncate(vp, (off_t)0, 0, NOCRED);
ump->um_truncate = ffs_truncate;
(void)ffs_truncate(vp, osize,
(void)ffs_truncate(vp, osize,
ffs_truncate(vp, 0, IO_EXT, cred);