When using m_dup(9) to copy more than MHLEN bytes of data, don't create an

mbuf chain that starts with a cluster containing just MHLEN bytes.  This
happened because m_dup called m_get or m_getcl depending on the amount of
data to copy, but then always set the size available in the first mbuf to
MHLEN.

Submitted by:	Matt Koivisto <mkoivisto at sandvine dot com>
Approved by:	jmg
Silence from:	rwatson (mentor)

sys/kern/uipc_mbuf.c

@@ -849,7 +849,8 @@ m_dup(struct mbuf *m, int how)
 				m_free(n);
 				goto nospace;
 			}
-			nsize = MHLEN;
+			if ((n->m_flags & M_EXT) == 0)
+				nsize = MHLEN;
 		}
 		n->m_len = 0;