unix: use m_freemp() when disposing unix socket buffers

The new unix/dgram uses m_nextpkt linkage, while the old unix/stream
uses m_next linkage.  This fixes memory leak.

Diagnosed by:		khng
Reviewed by:		khng, markj
PR:			279467
Fixes:			458f475df8
Differential Revision:	https://reviews.freebsd.org/D45478
MFC After:		1 week

sys/kern/uipc_usrreq.c

@@ -2190,7 +2190,7 @@ unp_disconnect(struct unpcb *unp, struct unpcb *unp2)
 
 	if (m != NULL) {
 		unp_scan(m, unp_freerights);
-		m_freem(m);
+		m_freemp(m);
 	}
 }
 
@@ -3280,7 +3280,7 @@ unp_dispose(struct socket *so)
 
 	if (m != NULL) {
 		unp_scan(m, unp_freerights);
-		m_freem(m);
+		m_freemp(m);
 	}
 }