netlink-socket: Work around kernel Netlink dump thread races.

The Linux kernel Netlink implementation has two races that cause problems
for processes that attempt to dump a table in a multithreaded manner.

The first race is in the structure of the kernel netlink_recv() function.
This function pulls a message from the socket queue and, if there is none,
reports EAGAIN:
	skb = skb_recv_datagram(sk, flags, noblock, &err);
	if (skb == NULL)
		goto out;
Only if a message is successfully read from the socket queue does the
function, toward the end, try to queue up a new message to be dumped:
	if (nlk->cb && atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2) {
		ret = netlink_dump(sk);
		if (ret) {
			sk->sk_err = ret;
			sk->sk_error_report(sk);
		}
	}
This means that if thread A reads a message from a dump, then thread B
attempts to read one before A queues up the next, B will get EAGAIN.  This
means that, following EAGAIN, B needs to wait until A returns to userspace
before it tries to read the socket again.  nl_dump_next() already does
this, using 'dump->status_seq' (although the need for it has never been
explained clearly, to my knowledge).

The second race is more serious.  Suppose thread X and thread Y both
simultaneously attempt to queue up a new message to be dumped, using the
call to netlink_dump() quoted above.  netlink_dump() begins with:
	mutex_lock(nlk->cb_mutex);

	cb = nlk->cb;
	if (cb == NULL) {
		err = -EINVAL;
		goto errout_skb;
	}
Suppose that X gets cb_mutex first and finds that the dump is complete.  It
will therefore, toward the end of netlink_dump(), clear nlk->cb to NULL to
indicate that no dump is in progress and release the mutex:
	nlk->cb = NULL;
	mutex_unlock(nlk->cb_mutex);
When Y grabs cb_mutex afterward, it will see that nlk->cb is NULL and
return -EINVAL as quoted above.  netlink_recv() stuffs -EINVAL in sk_err,
but that error is not reported immediately; instead, it is saved for the
next read from the socket.  Since Open vSwitch maintains a pool of Netlink
sockets, that next failure can crop up pretty much anywhere.  One of the
worst places for it to crop up is in the execution of a later transaction
(e.g. in nl_sock_transact_multiple__()), because userspace treats Netlink
transactions as idempotent and will re-execute them when socket errors
occur.  For a transaction that sends a packet, this causes packet
duplication, which we actually observed in practice.  (ENOBUFS should
actually cause transactions to be re-executed in many cases, but EINVAL
should not; this is a separate bug in the userspace netlink code.)

VMware-BZ: #1283188
Reported-and-tested-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>