Change uma_mtx to rwlock

[Steven Hartland]

----- Original Message ----- 
From: "Bryan Venteicher" <bryanv at daemoninthecloset.org>
> On Sat, Sep 27, 2014 at 8:42 PM, Steven Hartland <killing at multiplay.co.uk>
> wrote:
> 
>> Out of interest does that include ZFS and its UMA zones, as we're currently
>> investigating issues around this.
>>
>>
> Yes, I believe this would include ZFS's zones too

It would but I was more curious as it if you had seen the delay specifically on
the ZFS zone or if it was other zones which triggered the issue for you?

    Regards
    Steve