net: ipmr: fix static mfc/dev leaks on table destruction

When destroying an mrt table the static mfc entries and the static
devices are kept, which leads to devices that can never be destroyed
(because of refcnt taken) and leaked memory, for example:
unreferenced object 0xffff880034c144c0 (size 192):
  comm "mfc-broken", pid 4777, jiffies 4320349055 (age 46001.964s)
  hex dump (first 32 bytes):
    98 53 f0 34 00 88 ff ff 98 53 f0 34 00 88 ff ff  .S.4.....S.4....
    ef 0a 0a 14 01 02 03 04 00 00 00 00 01 00 00 00  ................
  backtrace:
    [<ffffffff815c1b9e>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff811ea6e0>] kmem_cache_alloc+0x190/0x300
    [<ffffffff815931cb>] ip_mroute_setsockopt+0x5cb/0x910
    [<ffffffff8153d575>] do_ip_setsockopt.isra.11+0x105/0xff0
    [<ffffffff8153e490>] ip_setsockopt+0x30/0xa0
    [<ffffffff81564e13>] raw_setsockopt+0x33/0x90
    [<ffffffff814d1e14>] sock_common_setsockopt+0x14/0x20
    [<ffffffff814d0b51>] SyS_setsockopt+0x71/0xc0
    [<ffffffff815cdbf6>] entry_SYSCALL_64_fastpath+0x16/0x7a
    [<ffffffffffffffff>] 0xffffffffffffffff

Make sure that everything is cleaned on netns destruction.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>