libceph: add __maybe_unused to DEFINE_CEPH_FEATURE

Avoid -Wunused-const-variable warnings for "make W=1".
Reported-by: NLeon Romanovsky <leonro@nvidia.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Reviewed-by: NLeon Romanovsky <leonro@nvidia.com>

include/linux/ceph/ceph_features.h

@@ -11,14 +11,14 @@
 #define CEPH_FEATURE_INCARNATION_2 (1ull<<57) // CEPH_FEATURE_SERVER_JEWEL
 
 #define DEFINE_CEPH_FEATURE(bit, incarnation, name)			\
-	static const uint64_t CEPH_FEATURE_##name = (1ULL<<bit);		\
-	static const uint64_t CEPH_FEATUREMASK_##name =			\
+	static const uint64_t __maybe_unused CEPH_FEATURE_##name = (1ULL<<bit);		\
+	static const uint64_t __maybe_unused CEPH_FEATUREMASK_##name =			\
 		(1ULL<<bit | CEPH_FEATURE_INCARNATION_##incarnation);
 
 /* this bit is ignored but still advertised by release *when* */
 #define DEFINE_CEPH_FEATURE_DEPRECATED(bit, incarnation, name, when) \
-	static const uint64_t DEPRECATED_CEPH_FEATURE_##name = (1ULL<<bit); \
-	static const uint64_t DEPRECATED_CEPH_FEATUREMASK_##name =		\
+	static const uint64_t __maybe_unused DEPRECATED_CEPH_FEATURE_##name = (1ULL<<bit);	\
+	static const uint64_t __maybe_unused DEPRECATED_CEPH_FEATUREMASK_##name =		\
 		(1ULL<<bit | CEPH_FEATURE_INCARNATION_##incarnation);
 
 /*