Remove a panic on vinum module unload: make sure to destroy all mutexes before freeing so that WITNESS doesn't dereference mutex data pointers and page fault.  It's now possible to unload vinum.ko with a GENERIC kernel on 5.0-CURRENT without panic.
Remove a panic on vinum module unload: make sure to destroy all mutexes
before freeing so that WITNESS doesn't dereference mutex data pointers
and page fault.  It's now possible to unload vinum.ko with a GENERIC
kernel on 5.0-CURRENT without panic.

Debugged/fixed with the aid of:	jake, grog

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