MFC r280850: Periodically wake up threads waiting for vmem(9) resources, so they could ask for resource reclamation again.
MFC r280850:
Periodically wake up threads waiting for vmem(9) resources, so they could
ask for resource reclamation again.

This is kind of dirty hack, but as last resort this is better then stuck
indefinitely because of KVA fragmentation, waiting until some random event
free something sufficient.  OpenSolaris also has this hack in its vmem(9).

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