sctp: Add buffer utilization fields to /proc/net/sctp/assocs

This patch adds the following fields to /proc/net/sctp/assocs output:

	- sk->sk_wmem_alloc as "wmema"	(transmit queue bytes committed)
	- sk->sk_wmem_queued as "wmemq"	(persistent queue size)
	- sk->sk_sndbuf as "sndbuf"	(size of send buffer in bytes)
	- sk->sk_rcvbuf as "rcvbuf"	(size of receive buffer in bytes)

When small DATA chunks containing 136 bytes data are sent the TX_QUEUE
(assoc->sndbuf_used) reaches a maximum of 40.9% of sk_sndbuf value when
peer.rwnd = 0. This was diagnosed from sk_wmem_alloc value reaching maximum
value of sk_sndbuf.

TX_QUEUE (assoc->sndbuf_used), sk_wmem_alloc and sk_wmem_queued values are
incremented in sctp_set_owner_w() for outgoing data chunks. Having access to
the above values in /proc/net/sctp/assocs will provide a better understanding
of SCTP buffer management.

With patch applied, example output when peer.rwnd = 0

where:
    ASSOC ffff880132298000 is sender
          ffff880125343000 is receiver

 ASSOC           SOCK            STY SST ST  HBKT ASSOC-ID TX_QUEUE RX_QUEUE \
ffff880132298000 ffff880124a0a0c0 2   1   3  29325    1      214656        0 \
ffff880125343000 ffff8801237d7700 2   1   3  36210    2           0   524520 \

UID   INODE LPORT  RPORT LADDRS <-> RADDRS       HBINT   INS  OUTS \
  0   25108 3455   3456  *10.4.8.3 <-> *10.5.8.3  7500     2     2 \
  0   27819 3456   3455  *10.5.8.3 <-> *10.4.8.3  7500     2     2 \

MAXRT T1X T2X RTXC   wmema   wmemq  sndbuf  rcvbuf
    4   0   0   72  525633  440320  524288  524288
    4   0   0    0       1       0  524288  524288
Signed-off-by: Dilip Daya <dilip.daya@hp.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>