Free UMA zones when a pass(4) instance goes away.

If the UMA zones are not freed, we get warnings about re-using the
sysctl variables associated with the UMA zones, and we're leaking
the other memory associated with the zone structures. e.g.:

sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.size)!
sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.flags)!
sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.bucket_size)!
sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.bucket_size_max)!
sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.keg.name)!
sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.keg.rsize)!
sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.keg.ppera)!
sysctl_warn_reuse: can't re-use a leaf (vm.uma.pass44.keg.ipers)!

Also, correctly clear the PASS_FLAG_ZONE_INPROG flag in
passcreatezone(). The way it was previously done, it would have
had set the flag and cleared all other flags that were set at
that point.

MFC after: 1 week
Sponsored by: Spectra Logic