Tokens should not be exposed to endusers for security reasons.

This patch updates the kernel so that /proc/net/mptcp does not include
information about the tokens when accessed by a regular user. To keep backward
compatibility with existing scripts, the tokens are listed as having value -1.

Here is a short example of the output of /proc/net/mptcp when the patch is
applied :

sudo cat /proc/net/mptcp
  sl  loc_tok  rem_tok  v6 local_address                         remote_address                        st ns tx_queue rx_queue inode
   0: 4ECCB36E ABCC0045  0 1500A8C0:CFEB                         2DE66882:1389                         01 04 0003E0E8:00000000 11435

as regular user:

cat /proc/net/mptcp
  sl  loc_tok  rem_tok  v6 local_address                         remote_address                        st ns tx_queue rx_queue inode
   0: FFFFFFFF FFFFFFFF  0 1500A8C0:CFEB                         2DE66882:1389                         01 04 00032E68:00000000 11435

A subsequent version of the kernel should probably update the format of the
/proc/net/mptcp file and remove the token information from this file.

Signed-off-by: Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be>
Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>