Locking fixes for xl(4)

[John Baldwin]

The patch below fixes several locking nits including the LOR involving 
taskqueue_drain()for xl(4).  It does not use taskqueue_stop() (it's #ifdef'd 
out) as it is not needed for correct operation, just as an optimization.  
Please test and let me know how it works.  Thanks.

http://www.FreeBSD.org/~jhb/patches/xl_locking.patch

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org