MFC r311963: Remove writability requirement for single-mbuf, contiguous-

range m_pulldown()

m_pulldown() only needs to determine if a mbuf is writable if it is going to
copy data into the data region of an existing mbuf. It does this to create a
contiguous data region in a single mbuf from multiple mbufs in the chain. If
the requested memory region is already contiguous and nothing needs to
change, the mbuf does not need to be writeable.

sys/kern/uipc_mbuf2.c

@@ -159,7 +159,7 @@ m_pulldown(struct mbuf *m, int off, int len, int *offp)
 	 * the target data is on <n, off>.
 	 * if we got enough data on the mbuf "n", we're done.
 	 */
-	if ((off == 0 || offp) && len <= n->m_len - off && writable)
+	if ((off == 0 || offp) && len <= n->m_len - off)
 		goto ok;
 
 	/*