drgn.helpers.linux.list: add list_count_nodes()

I've needed this many times, but there wasn't a corresponding function
in the kernel so I could never decide what to name it. Linux kernel
commit 4d70c74659d9 ("i915: Move list_count() to list.h as
list_count_nodes() for broader use") (in v6.3-rc1) fixed that problem
for me.

Signed-off-by: Omar Sandoval <osandov@osandov.com>

drgn/helpers/linux/list.py

@@ -19,6 +19,7 @@
     "hlist_empty",
     "hlist_for_each",
     "hlist_for_each_entry",
+    "list_count_nodes",
     "list_empty",
     "list_first_entry",
     "list_first_entry_or_null",
@@ -57,6 +58,15 @@ def list_is_singular(head: Object) -> bool:
     return next != head and next == head.prev
 
 
+def list_count_nodes(head: Object) -> int:
+    """
+    Return the number of nodes in a list.
+
+    :param head: ``struct list_head *``
+    """
+    return sum(1 for _ in list_for_each(head))
+
+
 def list_first_entry(head: Object, type: Union[str, Type], member: str) -> Object:
     """
     Return the first entry in a list.

tests/linux_kernel/helpers/test_list.py

@@ -9,6 +9,7 @@
     hlist_empty,
     hlist_for_each,
     hlist_for_each_entry,
+    list_count_nodes,
     list_empty,
     list_first_entry,
     list_first_entry_or_null,
@@ -56,6 +57,11 @@ def test_list_is_singular(self):
         self.assertFalse(list_is_singular(self.full))
         self.assertTrue(list_is_singular(self.singular))
 
+    def test_list_count_nodes(self):
+        self.assertEqual(list_count_nodes(self.empty), 0)
+        self.assertEqual(list_count_nodes(self.full), self.num_entries)
+        self.assertEqual(list_count_nodes(self.singular), 1)
+
     def test_list_first_entry(self):
         self.assertEqual(
             list_first_entry(self.full, "struct drgn_test_list_entry", "node"),