OpenZFS 9318 - vol_volsize_to_reservation does not account for raidz skip blocks

When a volume is created in a pool with raidz vdevs and
volblocksize != 128k, the volume can reference more space than is
reserved with the automatically calculated refreservation.  There
are two deficiencies in vol_volsize_to_reservation that contribute
to this:

  1) Skip blocks may be added to keep each allocation a multiple
     of parity + 1. This is the dominating factor when volblocksize
     is close to 2^ashift.

  2) raidz deflation for 128 KB blocks is different for most other
     block sizes.

See "The theory of raidz space accounting" comment in
libzfs_dataset.c for a full explanation.

Authored by: Mike Gerdts <[email protected]>
Reviewed by: Richard Elling <[email protected]>
Reviewed by: Sanjay Nadkarni <[email protected]>
Reviewed by: Jerry Jelinek <[email protected]>
Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: Kody Kantor <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Approved by: Dan McDonald <[email protected]>
Ported-by: Mike Gerdts <[email protected]>

Porting Notes:
* ZTS: wait for zvols to exist before writing
* ZTS: use log_must_busy with {zpool|zfs} destroy

OpenZFS-issue: https://www.illumos.org/issues/9318
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/b73ccab0
Closes #8973

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