Zpool can start allocating from metaslab before TRIMs have completed

When doing a manual TRIM on a zpool, the metaslab being TRIMmed is
potentially re-enabled before all queued TRIM zios for that metaslab
have completed. Since TRIM zios have the lowest priority, it is 
possible to get into a situation where allocations occur from the 
just re-enabled metaslab and cut ahead of queued TRIMs to the same 
metaslab.  If the ranges overlap, this will cause corruption.

We were able to trigger this pretty consistently with a small single 
top-level vdev zpool (i.e. small number of metaslabs) with heavy 
parallel write activity while performing a manual TRIM against a 
somewhat 'slow' device (so TRIMs took a bit of time to complete). 
With the patch, we've not been able to recreate it since. It was on 
illumos, but inspection of the OpenZFS trim code looks like the 
relevant pieces are largely unchanged and so it appears it would be 
vulnerable to the same issue.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jason King <jking@racktopsystems.com>
Illumos-issue: https://www.illumos.org/issues/15939
Closes #15395 

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