It turns out that devfs_close() does a dev_refthread() before invoking

device specific d_close(), which makes subsequent destroy_dev() being
blocked in the "devdrn" loop.

This bandaid should fix the smbfs hang/crashing observed on -CURRENT since
the introduction of sys/kern/kern_conf.c:1.199:

 	# mount_smbfs -I server //server/share /mnt
 	Password:
 	[hang]

Reviewed by:	bp
See also:	http://lists.freebsd.org/pipermail/cvs-src/2006-November/071379.html

sys/netsmb/smb_dev.c

@@ -175,6 +175,7 @@ nsmb_dev_close(struct cdev *dev, int flag, int fmt, struct thread *td)
 */
 	dev->si_drv1 = NULL;
 	free(sdp, M_NSMBDEV);
+	dev_relthread(dev);	/* XXX dealing with si_threadcount */
 	destroy_dev(dev);
 	splx(s);
 	return 0;