Refactor dnode dirty context from dbuf_dirty

* Add dedicated donde_set_dirtyctx routine.
* Add empty dirty record on destroy assertion.
* Make much more extensive use of the SET_ERROR macro.

Reviewed-by: Will Andrews <wca@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9924

module/zfs/zap_leaf.c

@@ -467,7 +467,7 @@ zap_leaf_lookup_closest(zap_leaf_t *l,
 		}
 	}
 
-	return (bestcd == -1U ? ENOENT : 0);
+	return (bestcd == -1U ? SET_ERROR(ENOENT) : 0);
 }
 
 int