lockdep: Mark chain_hlock_class_idx() with __maybe_unused

When chain_hlock_class_idx() is unused, it prevents kernel builds with
clang, `make W=1` and CONFIG_WERROR=y, CONFIG_LOCKDEP=y and
CONFIG_PROVE_LOCKING=n:

kernel/locking/lockdep.c:435:28: error: unused function 'chain_hlock_class_idx' [-Werror,-Wunused-function]

Fix this by marking it with __maybe_unused.

See also commit 6863f56 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

[Boqun: add more config information of the error]

Signed-off-by: Andy Shevchenko <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: andy-shev

kernel/locking/lockdep.c

@@ -430,7 +430,7 @@ static inline u16 hlock_id(struct held_lock *hlock)
 	return (hlock->class_idx | (hlock->read << MAX_LOCKDEP_KEYS_BITS));
 }
 
-static inline unsigned int chain_hlock_class_idx(u16 hlock_id)
+static inline __maybe_unused unsigned int chain_hlock_class_idx(u16 hlock_id)
 {
 	return hlock_id & (MAX_LOCKDEP_KEYS - 1);
 }