checkpatch: Avoid catastrophic backtracking.

As Frode Nordahl points out in [0], it is possible for the
python regex module to enter a case of catastrophic backtracking
which causes oscillation between states and hangs the checkpatch
script.

One suggested solution to these cases is to use an anchor[1] in
the regex, which should force the backtrack to exit early.
However, when I tested this, it didn't seem to improve anything
(since the start is already anchored, and trying to anchor the
end results in the same hang).

Instead, we explicitly check that the line ends with '\\' before
trying to match on the 'if-inside-a-macro' check.  A new check
is added to catch the case in checkpatch.

0: https://mail.openvswitch.org/pipermail/ovs-dev/2021-August/386881.html
1: https://stackoverflow.com/questions/22072406/preventing-any-backtracking-in-regex-past-a-specific-pattern

Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

utilities/checkpatch.py

@@ -274,9 +274,13 @@ def if_and_for_end_with_bracket_check(line):
         if not balanced_parens(line):
             return True
 
-        if __regex_ends_with_bracket.search(line) is None and \
-           __regex_if_macros.match(line) is None:
-            return False
+        if __regex_ends_with_bracket.search(line) is None:
+            if line.endswith("\\") and \
+               __regex_if_macros.match(line) is not None:
+                return True
+            else:
+                return False
+
     if __regex_conditional_else_bracing.match(line) is not None:
         return False
     if __regex_conditional_else_bracing2.match(line) is not None: