vdev_draid_min_asize() ignores reserved space

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.

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 openzfs#11459
Signed-off-by: Matthew Ahrens <[email protected]>

module/zfs/vdev_draid.c

@@ -1132,7 +1132,8 @@ vdev_draid_min_asize(vdev_t *vd)
 
 	ASSERT3P(vd->vdev_ops, ==, &vdev_draid_ops);
 
-	return ((vd->vdev_min_asize + vdc->vdc_ndisks - 1) / (vdc->vdc_ndisks));
+	return (VDEV_DRAID_REFLOW_RESERVE +
+	    (vd->vdev_min_asize + vdc->vdc_ndisks - 1) / (vdc->vdc_ndisks));
 }
 
 /*