OpenZFS 7885 - zpool list can report 16.0e for expandsz

OpenZFS 7885 - zpool list can report 16.0e for expandsz

Authored by: Steven Hartland <steven.hartland@multiplay.co.uk>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Gordon Ross <gordon.w.ross@gmail.com>

When a member of a RAIDZ has been replaced with a device smaller than
the original, then the top level vdev can report its expand size as
16.0E.
The reduced child asize causes the RAIDZ to have a vdev_asize lower than
its vdev_max_asize which then results in an underflow during the
calculation of the parents expand size.
Fix this by updating the vdev_asize if it shrinks, which is already
protected by a check against vdev_min_asize so should always be safe.
Also for RAIDZ vdevs, ensure that the sum of their child vdev_min_asize
is always greater than the parents vdev_min_size.

Reviewed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7885
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/bb0dbaa
Closes #5963