net: sctp: fix memory leak in auth key management

A very minimal and simple user space application allocating an SCTP
socket, setting SCTP_AUTH_KEY setsockopt(2) on it and then closing
the socket again will leak the memory containing the authentication
key from user space:

unreferenced object 0xffff8800837047c0 (size 16):
  comm "a.out", pid 2789, jiffies 4296954322 (age 192.258s)
  hex dump (first 16 bytes):
    01 00 00 00 04 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff816d7e8e>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff811c88d8>] __kmalloc+0xe8/0x270
    [<ffffffffa0870c23>] sctp_auth_create_key+0x23/0x50 [sctp]
    [<ffffffffa08718b1>] sctp_auth_set_key+0xa1/0x140 [sctp]
    [<ffffffffa086b383>] sctp_setsockopt+0xd03/0x1180 [sctp]
    [<ffffffff815bfd94>] sock_common_setsockopt+0x14/0x20
    [<ffffffff815beb61>] SyS_setsockopt+0x71/0xd0
    [<ffffffff816e58a9>] system_call_fastpath+0x12/0x17
    [<ffffffffffffffff>] 0xffffffffffffffff

This is bad because of two things, we can bring down a machine from
user space when auth_enable=1, but also we would leave security sensitive
keying material in memory without clearing it after use. The issue is
that sctp_auth_create_key() already sets the refcount to 1, but after
allocation sctp_auth_set_key() does an additional refcount on it, and
thus leaving it around when we free the socket.

Fixes: 65b07e5d ("[SCTP]: API updates to suport SCTP-AUTH extensions.")
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>