Change uma_mtx to rwlock

[Bryan Venteicher]

Hi,

I'd appreciate some comments attached patch that changes the uma_mtx to a
rwlock.

At $JOB, we have machines with ~400GB RAM, with much of that being
allocated through UMA zones. We've observed that timeouts were sometimes
unexpectedly delayed by a half second or more. We tracked one of the
reasons for this down to when the page daemon was running, calling
uma_reclaim() -> zone_foreach(). zone_foreach() holds the uma_mtx while
zone_drain()'ing each zone. If uma_timeout() fires, it will block on the
uma_mtx when it tries to zone_timeout() each zone.
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