Fix m_freem_vec() to actually traverse the mbuf chain. This avoids

double free's and an infinite loop.

CID:		1834
Found by:	Coverity Prevent (tm)

sys/dev/cxgb/sys/mvec.h

@@ -135,6 +135,7 @@ m_freem_vec(struct mbuf *m)
 			mb_free_ext(m);
 		else
 			uma_zfree(zone_mbuf, m);
+		m = n;
 	}
 }