kcm: fix /proc memory leak

Every open of /proc/net/kcm leaks 16 bytes of memory as is reported by
kmemleak:
unreferenced object 0xffff88059c0e3458 (size 192):
  comm "cat", pid 1401, jiffies 4294935742 (age 310.720s)
  hex dump (first 32 bytes):
    28 45 71 96 05 88 ff ff 00 10 00 00 00 00 00 00  (Eq.............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff8156a2de>] kmem_cache_alloc_trace+0x16e/0x230
    [<ffffffff8162a479>] seq_open+0x79/0x1d0
    [<ffffffffa0578510>] kcm_seq_open+0x0/0x30 [kcm]
    [<ffffffff8162a479>] seq_open+0x79/0x1d0
    [<ffffffff8162a8cf>] __seq_open_private+0x2f/0xa0
    [<ffffffff81712548>] seq_open_net+0x38/0xa0
...

It is caused by a missing free in the ->release path. So fix it by
providing seq_release_net as the ->release method.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Fixes: cd6e111b (kcm: Add statistics and proc interfaces)
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Tom Herbert <tom@herbertland.com>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>