kcsan: Never set up watchpoints on NULL pointers

Avoid setting up watchpoints on NULL pointers, as otherwise we would
crash inside the KCSAN runtime (when checking for value changes) instead
of the instrumented code.

Because that may be confusing, skip any address less than PAGE_SIZE.
Reviewed-by: NDmitry Vyukov <dvyukov@google.com>
Signed-off-by: NMarco Elver <elver@google.com>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>

kernel/kcsan/encoding.h

@@ -48,7 +48,11 @@
 
 static inline bool check_encodable(unsigned long addr, size_t size)
 {
-	return size <= MAX_ENCODABLE_SIZE;
+	/*
+	 * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
+	 * pointer deref inside KCSAN.
+	 */
+	return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
 }
 
 static inline long