DLPX-86794 sdb: zio broken after upstream ZFS commit (#331)

Our `zio` command broke from the commit introduced by the following
upstream PR: openzfs/zfs#14948

The problem is that the PR removed `io_parent_count` from `struct zio`
and we are using this field to find the top-level ZIOs (e.g. ZIOs with
no parents) in the system.

This patch changes our SBD command to check whether the `io_parent_list`
is populated instead.

= Github Issue Tracker Automation

Closes #328

sdb/commands/zfs/zio.py

@@ -124,6 +124,14 @@ def from_zio(self, zio: drgn.Object) -> Iterable[drgn.Object]:
                 yield from self.from_zio(c.zl_parent)
         self.level -= 1
 
+    @staticmethod
+    def zio_has_parents(zio: drgn.Object) -> bool:
+        parent_list = zio.io_parent_list.list_head.address_of_()
+        first_parent = parent_list.next
+        if parent_list != first_parent:
+            return True
+        return False
+
     def no_input(self) -> drgn.Object:
         if self.args.parents:
             raise sdb.CommandInvalidInputError(
@@ -136,5 +144,5 @@ def no_input(self) -> drgn.Object:
             [sdb.Walk(), sdb.Cast(["zio_t *"])],
         )
         for zio in zios:
-            if zio.io_parent_count == 0:
+            if not self.zio_has_parents(zio):
                 yield from self.from_zio(zio)