Avoid overfow in vtruncbuf()

Using daddr_t instead of int avoids trunclbn to become negative when it
shouldn't.
This isssue was found by running syzkaller.

Reviewed by:		mckusick, kib, markj
MFC after:		1 week
Differential Revision:	https://reviews.freebsd.org/D18763

sys/kern/vfs_subr.c

@@ -1858,7 +1858,7 @@ vtruncbuf(struct vnode *vp, struct ucred *cred, off_t length, int blksize)
 {
 	struct buf *bp, *nbp;
 	int anyfreed;
-	int trunclbn;
+	daddr_t trunclbn;
 	struct bufobj *bo;
 
 	CTR5(KTR_VFS, "%s: vp %p with cred %p and block %d:%ju", __func__,