Link._submit: Pass is_self to constructor.

This was causing deletes to be issued to the `LinksByUrl` column
family for all self posts. Self posts that were left in an
inconsistent state due to dropped double commits ended up reading this
row which had several hundred thousand tombstones, which lead to
Cassandra performance issues.

r2/r2/models/link.py

@@ -212,18 +212,25 @@ def _submit(cls, is_self, title, content, author, sr, ip,
                 spam=False, sendreplies=True):
         from r2.models import admintools
 
-        l = cls(_ups=1,
-                title=title,
-                url=TEMPORARY_SELFPOST_URL if is_self else content,
-                _spam=spam,
-                author_id=author._id,
-                sendreplies=sendreplies,
-                sr_id=sr._id,
-                lang=sr.lang,
-                ip=ip,
-                comment_tree_version=cls._choose_comment_tree_version())
+        l = cls(
+            _ups=1,
+            title=title,
+            url=TEMPORARY_SELFPOST_URL if is_self else content,
+            _spam=spam,
+            author_id=author._id,
+            sendreplies=sendreplies,
+            sr_id=sr._id,
+            lang=sr.lang,
+            ip=ip,
+            comment_tree_version=cls._choose_comment_tree_version(),
+            is_self=is_self,
+        )
 
         l._commit()
+        # Note: this does not provide atomicity, so for self posts when
+        # a request dies after the previous line but before `set_type`,
+        # you will have a link whose URL is literally 'self', and whose
+        # selftext will not be set.
         l.set_type(is_self, content)
 
         LinksByAccount.add_link(author, l)