Add a coccinelle script to look for {inc,dec}rements in log_debug

We want to forbid this pattern since, unlike the other log_*()
macros, log_debug() conditionally evaluates its arguments only if
debug-level logging is enabled.  Thus, a call to
    log_debug("%d", x++);
will only increment x if debugging logs are enabled, which is
probably not what the programmer intended.

One bug caused by this pattern was #30628.

This script detects log_debug( ) calls with any of E++, E--, ++E,
or --E in their arguments, where E is an arbitrary expression.

Closes ticket 30743.

scripts/coccinelle/debugmm.cocci

@@ -0,0 +1,29 @@
+// Look for use of expressions with side-effects inside of debug logs.
+//
+// This script detects expressions like ++E, --E, E++, and E-- inside of
+// calls to log_debug().
+//
+// The log_debug() macro exits early if debug logging is not enabled,
+// potentially causing problems if its arguments have side-effects.
+
+@@
+expression E;
+@@
+*log_debug(... , <+...  --E ...+>, ... );
+
+
+@@
+expression E;
+@@
+*log_debug(... , <+...  ++E ...+>, ... );
+
+@@
+expression E;
+@@
+*log_debug(... , <+...  E-- ...+>, ... );
+
+
+@@
+expression E;
+@@
+*log_debug(... , <+...  E++ ...+>, ... );