rhashtable: Move mutex_is_held under PROVE_LOCKING

The rhashtable function mutex_is_held is only used when PROVE_LOCKING
is enabled.  This patch makes the mutex_is_held field in rhashtable
optional depending on PROVE_LOCKING.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index fb298e9d..96ce8ce 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -65,7 +65,9 @@
 						 size_t new_size);
 	bool			(*shrink_decision)(const struct rhashtable *ht,
 						   size_t new_size);
+#ifdef CONFIG_PROVE_LOCKING
 	int			(*mutex_is_held)(void);
+#endif
 };
 
 /**