IPv4/IPv6 sysctl unregistration deadlock

Ben Greear reported sporadically hanging ip processes when adding or
removing MACVLAN devices, which seem to be caused by a race between
sysctl handling and device notifiers.

What is happening is:

Process 1:

- "ip" deletes a macvlan device (or any other kind of device gets
   removed for whatever reason)
- netdev notifier chain notifies IPv6 addrconf while holding the RTNL

Process 2:

- a different process writes something to /proc/sys/net/ipv6/forwarding
- sysctl table is marked as busy, addrconf_sysctl_forward() is invoked
- tries to take rtnl, waits for process 1

Process 1:

- IPv6 begins sysctl unregistration, which is deferred using a
   completion until the table is not busy anymore (=> waiting for
   process 2)

At this point both processes are deadlocked. Judging by a quick look,
IPv4 seems to have the exact same problem.

An easy fix would be to keep track of whether sysctl unregistration
is in progress in IPv4/IPv6 and ignore new requests from that point
on. Its not very elegant though, so I was wondering whether anyone
has a better suggestion.