netlink: Re-add locking to netlink_lookup() and seq walker

netlink: Re-add locking to netlink_lookup() and seq walker

The synchronize_rcu() in netlink_release() introduces unacceptable
latency. Reintroduce minimal lookup so we can drop the
synchronize_rcu() until socket destruction has been RCUfied.

Cc: David S. Miller <[email protected]>
Cc: Eric Dumazet <[email protected]>
Reported-by: Steinar H. Gunderson <[email protected]>
Reported-and-tested-by: Heiko Carstens <[email protected]>
Signed-off-by: Thomas Graf <[email protected]>
Signed-off-by: David S. Miller <[email protected]>