netrom: fix info-leak in nr_write_internal()

Simon Kapadia reported the following issue:

<quote>

The Online Amateur Radio Community (OARC) has recently been experimenting
with building a nationwide packet network in the UK.
As part of our experimentation, we have been testing out packet on 300bps HF,
and playing with net/rom.  For HF packet at this baud rate you really need
to make sure that your MTU is relatively low; AX.25 suggests a PACLEN of 60,
and a net/rom PACLEN of 40 to go with that.
However the Linux net/rom support didn't work with a low PACLEN;
the mkiss module would truncate packets if you set the PACLEN below about 200 or so, e.g.:

Apr 19 14:00:51 radio kernel: [12985.747310] mkiss: ax1: truncating oversized transmit packet!

This didn't make any sense to me (if the packets are smaller why would they
be truncated?) so I started investigating.
I looked at the packets using ethereal, and found that many were just huge
compared to what I would expect.
A simple net/rom connection request packet had the request and then a bunch
of what appeared to be random data following it:

</quote>

Simon provided a patch that I slightly revised:
Not only we must not use skb_tailroom(), we also do
not want to count NR_NETWORK_LEN twice.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Co-Developed-by: Simon Kapadia <szymon@kapadia.pl>
Signed-off-by: Simon Kapadia <szymon@kapadia.pl>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Simon Kapadia <szymon@kapadia.pl>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230524141456.1045467-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/netrom/nr_subr.c

@@ -123,7 +123,7 @@ void nr_write_internal(struct sock *sk, int frametype)
 	unsigned char  *dptr;
 	int len, timeout;
 
-	len = NR_NETWORK_LEN + NR_TRANSPORT_LEN;
+	len = NR_TRANSPORT_LEN;
 
 	switch (frametype & 0x0F) {
 	case NR_CONNREQ:
@@ -141,7 +141,8 @@ void nr_write_internal(struct sock *sk, int frametype)
 		return;
 	}
 
-	if ((skb = alloc_skb(len, GFP_ATOMIC)) == NULL)
+	skb = alloc_skb(NR_NETWORK_LEN + len, GFP_ATOMIC);
+	if (!skb)
 		return;
 
 	/*
@@ -149,7 +150,7 @@ void nr_write_internal(struct sock *sk, int frametype)
 	 */
 	skb_reserve(skb, NR_NETWORK_LEN);
 
-	dptr = skb_put(skb, skb_tailroom(skb));
+	dptr = skb_put(skb, len);
 
 	switch (frametype & 0x0F) {
 	case NR_CONNREQ: