netgraph/ng_checksum: Fix double free error

m_pullup(9) frees the mbuf(9) chain in the case of an allocation error.
The mbuf chain must not be freed again in this case.

PR:		255874
Submitted by:	<lylgood@foxmail.com>
Approved by:	markj
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D30273

sys/netgraph/ng_checksum.c

@@ -682,9 +682,9 @@ ng_checksum_rcvdata(hook_p hook, item_p item)
 	return (error);
 
 done:
+	NG_FREE_M(m);
 drop:
 	NG_FREE_ITEM(item);
-	NG_FREE_M(m);
 
 	priv->stats.dropped++;