Enables vdev traces for ZIL writes, and then only issues flushes to
things that were written to.
This simplifies a few things. We no longer have to extract the toplevel
vdevs to flush from the block pointer; instead we just look at what was
written. The vdev tree remains as a means to defer flushes to the next
lwb, which means a bit more copying trees, but also means we no longer
have to lock the tree.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
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