Create zap for root vdev

And add it to the AVZ, this is not backwards compatible with older pools
due to an assertion in spa_sync() that verifies the number of ZAPs of
all vdevs matches the number of ZAPs in the AVZ.

Granted, the assertion only applies to #DEBUG builds - still, a feature
flag is introduced to avoid the assertion, com.klarasystems:vdev_zaps_v2

Notably, this allows to get/set properties on the root vdev:

    % zpool set user:prop=value <pool> root-0

Before this commit, it was already possible to get/set properties on
top-level vdevs with the syntax <type>-<vdev_id> (e.g. mirror-0):

    % zpool set user:prop=value <pool> mirror-0

This syntax also applies to the root vdev as it is is of type 'root'
with a vdev_id of 0, root-0. The keyword 'root' as an alias for
'root-0'.

The following tests have been added:

    - zpool get all properties from root vdev
    - zpool set a property on root vdev
    - verify root vdev ZAP is created

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Wing <rob.wing@klarasystems.com>
Sponsored-by: Seagate Technology
Submitted-by: Klara, Inc.
Closes #14405

man/man7/zpool-features.7

@@ -858,6 +858,22 @@ by user and group.
 \*[instant-never]
 \*[remount-upgrade]
 .
+.feature com.klarasystems vdev_zaps_v2 no
+This feature creates a ZAP object for the root vdev.
+.Pp
+This feature becomes active after the next
+.Nm zpool Cm import
+or
+.Nm zpool reguid .
+.
+Properties can be retrieved or set on the root vdev using
+.Nm zpool Cm get
+and
+.Nm zpool Cm set
+with
+.Sy root
+as the vdev name which is an alias for
+.Sy root-0 .
 .feature org.openzfs zilsaxattr yes extensible_dataset
 This feature enables
 .Sy xattr Ns = Ns Sy sa