[PPP]: Fix output buffer size in ppp_decompress_frame().

This patch addresses the issue with "osize too small" errors in mppe
encryption.  The patch fixes the issue with wrong output buffer size
being passed to ppp decompression routine.

--------------------
As pointed out by Suresh Mahalingam, the issue addressed by
ppp-fix-osize-too-small-errors-when-decoding patch is not fully resolved yet.
The size of allocated output buffer is correct, however it size passed to
ppp->rcomp->decompress in ppp_generic.c if wrong. The patch fixes that.
--------------------
Signed-off-by: NKonstantin Sharlaimov <konstantin.sharlaimov@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ppp_generic.c

@@ -1726,7 +1726,7 @@ ppp_decompress_frame(struct ppp *ppp, struct sk_buff *skb)
 		}
 		/* the decompressor still expects the A/C bytes in the hdr */
 		len = ppp->rcomp->decompress(ppp->rc_state, skb->data - 2,
-				skb->len + 2, ns->data, ppp->mru + PPP_HDRLEN);
+				skb->len + 2, ns->data, obuff_size);
 		if (len < 0) {
 			/* Pass the compressed frame to pppd as an
 			   error indication. */