Teach the (gpt)zfsboot and zfsloader raidz code to use its buffers more efficiently.
Teach the (gpt)zfsboot and zfsloader raidz code to use its buffers
more efficiently.

Before this patch, in the worst case memory use would increase
exponentially on the number of drives in the raidz vdev.

Submitted by:	Matt Reimer <mattjreimer@gmail.com>
Sponsored by:	VPOP Technologies, Inc.
Silence from:	dfr

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