Have TD_LOCKS_DEC() assert that td_locks is positive.

This makes it easier to catch lock accounting bugs, since the problem
is otherwise only detected upon a return to user mode (or never, for
kernel threads).

Reviewed by:	cem
MFC after:	1 week
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D14896

sys/sys/proc.h

@@ -381,7 +381,10 @@ do {									\
 } while (0)
 
 #define	TD_LOCKS_INC(td)	((td)->td_locks++)
-#define	TD_LOCKS_DEC(td)	((td)->td_locks--)
+#define	TD_LOCKS_DEC(td) do {						\
+	KASSERT((td)->td_locks > 0, ("thread %p owns no locks", (td)));	\
+	(td)->td_locks--;						\
+} while (0)
 #else
 #define	THREAD_LOCKPTR_ASSERT(td, lock)