It was broken for several reasons:
* VOP_UNLOCK lost an argument in 13.0.  So OpenZFS should be using
  VOP_UNLOCK1, but a few direct calls to VOP_UNLOCK snuck in.
* The location of the zlib header moved in 13.0 and 12.1.  We can drop
  support for building on 12.0, which is EoL.
* knlist_init lost an argument in 13.0.  OpenZFS change 9d088740
  assumed 13.0 or later.
* FreeBSD 13.0 added copy_file_range, and OpenZFS change 67a1b037


  assumed 13.0 or later.

Sponsored-by: Axcient
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #15551