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

tests/runfiles/common.run

@@ -394,7 +394,7 @@ tags = ['functional', 'cli_root', 'zpool_export']
 
 [tests/functional/cli_root/zpool_get]
 tests = ['zpool_get_001_pos', 'zpool_get_002_pos', 'zpool_get_003_pos',
-    'zpool_get_004_neg', 'zpool_get_005_pos']
+    'zpool_get_004_neg', 'zpool_get_005_pos', 'vdev_get_001_pos']
 tags = ['functional', 'cli_root', 'zpool_get']
 
 [tests/functional/cli_root/zpool_history]
@@ -482,7 +482,7 @@ tags = ['functional', 'cli_root', 'zpool_scrub']
 
 [tests/functional/cli_root/zpool_set]
 tests = ['zpool_set_001_pos', 'zpool_set_002_neg', 'zpool_set_003_neg',
-    'zpool_set_ashift', 'zpool_set_features']
+    'zpool_set_ashift', 'zpool_set_features', 'vdev_set_001_pos']
 tags = ['functional', 'cli_root', 'zpool_set']
 
 [tests/functional/cli_root/zpool_split]