SCTP: Free the per-net sysctl table on net exit. v2

Per-net sysctl table needs to be explicitly freed at
net exit.  Otherwise we see the following with kmemleak:

unreferenced object 0xffff880402d08000 (size 2048):
  comm "chrome_sandbox", pid 18437, jiffies 4310887172 (age 9097.630s)
  hex dump (first 32 bytes):
    b2 68 89 81 ff ff ff ff 20 04 04 f8 01 88 ff ff  .h...... .......
    04 00 00 00 a4 01 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff815b4aad>] kmemleak_alloc+0x21/0x3e
    [<ffffffff81110352>] slab_post_alloc_hook+0x28/0x2a
    [<ffffffff81113fad>] __kmalloc_track_caller+0xf1/0x104
    [<ffffffff810f10c2>] kmemdup+0x1b/0x30
    [<ffffffff81571e9f>] sctp_sysctl_net_register+0x1f/0x72
    [<ffffffff8155d305>] sctp_net_init+0x100/0x39f
    [<ffffffff814ad53c>] ops_init+0xc6/0xf5
    [<ffffffff814ad5b7>] setup_net+0x4c/0xd0
    [<ffffffff814ada5e>] copy_net_ns+0x6d/0xd6
    [<ffffffff810938b1>] create_new_namespaces+0xd7/0x147
    [<ffffffff810939f4>] copy_namespaces+0x63/0x99
    [<ffffffff81076733>] copy_process+0xa65/0x1233
    [<ffffffff81077030>] do_fork+0x10b/0x271
    [<ffffffff8100a0e9>] sys_clone+0x23/0x25
    [<ffffffff815dda73>] stub_clone+0x13/0x20
    [<ffffffffffffffff>] 0xffffffffffffffff

I fixed the spelling of sysctl_header so the code actually
compiles. -- EWB.

Reported-by: Martin Mokrejs <mmokrejs@fold.natur.cuni.cz>
Signed-off-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>