net: ppp: access ppp->nextseq only if CONFIG_PPP_MULTILINK is defined

Commit d762d038497c9df51c19fcbe69b094b3bf8e5568 resets the counter holding the
next sequence number for multilink PPP fragments to zero whenever the
SC_MULTILINK flag is set. However, this counter only exists if
CONFIG_PPP_MULTILINK is defined. Consequently, the new code has to be enclosed
within #ifdef CONFIG_PPP_MULTILINK ... #endif.

Signed-off-by: Christoph Schulz <develop@kristov.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -655,8 +655,10 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			break;
 		ppp_lock(ppp);
 		cflags = ppp->flags & ~val;
+#ifdef CONFIG_PPP_MULTILINK
 		if (!(ppp->flags & SC_MULTILINK) && (val & SC_MULTILINK))
 			ppp->nextseq = 0;
+#endif
 		ppp->flags = val & SC_FLAG_BITS;
 		ppp_unlock(ppp);
 		if (cflags & SC_CCP_OPEN)