Clean up large dnode code

Resolves issues discovered when porting to OpenZFS.

* Lint warnings.
* Made dnode_move_impl() large dnode aware.  This
  functionality is currently unused on Linux.

Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #6262

cmd/ztest/ztest.c

@@ -4986,6 +4986,7 @@ ztest_dmu_commit_callbacks(ztest_ds_t *zd, uint64_t id)
  * are consistent what was stored in the block tag when it was created,
  * and that its unused bonus buffer space has not been overwritten.
  */
+/* ARGSUSED */
 void
 ztest_verify_dnode_bt(ztest_ds_t *zd, uint64_t id)
 {