vdev_draid_min_asize() ignores reserved space

[ahrens]

Motivation and Context

vdev_draid_min_asize() returns the minimum size of a child vdev. This
is used when determining if a disk is big enough to replace a child.
It's also used by zdb to determine how big of a child to make to test
replacement.

`vdev_draid_min_asize() says that the child’s asize has to be at least 1/Nth of
the entire draid’s asize, which is the same logic as raidz. However, this
contradicts the code in vdev_draid_open(), which calculates the draid’s asize
based on a reduced child size:

An additional 32MB of scratch space is reserved at the end of each child for
use by the dRAID expansion feature

So the problem is that you can replace a draid disk with one that’s
vdev_draid_min_asize(), but it actually needs to be larger to accomodate the
additional 32MB. The replacement is allowed and everything works at first
(since the reserved space is at the end, and we don’t try to use it yet), but
when you try to close and reopen the pool, vdev_draid_open() calculates a
smaller asize for the draid, because of the smaller leaf, which is not allowed.

I think the confusion is that vdev_draid_min_asize() is correctly returning the
amount of required allocatable space in a leaf, but the actual size of the
leaf needs to be at least 32MB more than that. ztest_vdev_attach_detach()
assumes that it can attach that size of device, and it actually can (the
kernel/libzpool accepts it), but it then later causes zdb to not be able to open the pool.

Description

This commit changes vdev_draid_min_asize() to return the required size of the
leaf, not the size that draid will make available to the metaslab allocator.

Closes #11459

How Has This Been Tested?

zloop

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.