fixup to "Elide locking db_data and db_mtx where it seems safe"

On Linux, assert_db_data_contents_locked can't take the db pointer as a
const pointer, because the rwsem_is_locked function requires mutable
access .

Author: asomers

include/sys/dbuf.h

@@ -338,7 +338,11 @@ void assert_db_data_addr_locked(const dmu_buf_impl_t *db);
  * and by no other thread.  Either it must be locked, or in a state where
  * locking is not required.
  */
-void assert_db_data_contents_locked(const dmu_buf_impl_t *db, boolean_t writer);
+#ifdef __linux__
+void assert_db_data_contents_locked(dmu_buf_impl_t *db, boolean_t wr);
+#else
+void assert_db_data_contents_locked(const dmu_buf_impl_t *db, boolean_t wr);
+#endif
 
 #define	DBUF_HASH_MUTEX(h, idx) \
 	(&(h)->hash_mutexes[(idx) & ((h)->hash_mutex_mask)])

module/zfs/dbuf.c

@@ -297,7 +297,11 @@ assert_db_data_addr_locked(const dmu_buf_impl_t *db)
 }
 
 void
+#ifdef __linux__
+assert_db_data_contents_locked(dmu_buf_impl_t *db, boolean_t writer)
+#else
 assert_db_data_contents_locked(const dmu_buf_impl_t *db, boolean_t writer)
+#endif
 {
 	/* 
 	 * db_rwlock protects indirect blocks and the data block of the meta