netlink-socket: Work around upstream kernel Netlink bug.

The upstream kernel net/netlink/af_netlink.c netlink_recvmsg() contains the
following code to refill the Netlink socket buffer with more dump skbs
while a dump is in progress:

	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);
		}
	}

The netlink_dump() function that this calls returns a negative number on
error, the convention used throughout the kernel, and thus sk->sk_err
receives a negative value on error.

However, sk->sk_err is supposed to contain either 0 or a positive errno
value, as one can see from a quick "grep" through net for 'sk_err =', e.g.:

    ipv4/tcp.c:2067:		sk->sk_err = ECONNRESET;
    ipv4/tcp.c:2069:		sk->sk_err = ECONNRESET;
    ipv4/tcp_input.c:4106:		sk->sk_err = ECONNREFUSED;
    ipv4/tcp_input.c:4109:		sk->sk_err = EPIPE;
    ipv4/tcp_input.c:4114:		sk->sk_err = ECONNRESET;
    netlink/af_netlink.c:741:			sk->sk_err = ENOBUFS;
    netlink/af_netlink.c:1796:			sk->sk_err = ENOBUFS;
    packet/af_packet.c:2476:		sk->sk_err = ENETDOWN;
    unix/af_unix.c:341:			other->sk_err = ECONNRESET;
    unix/af_unix.c:407:				skpair->sk_err = ECONNRESET;

The result is that the next attempt to receive from the socket will return
the error to userspace with the wrong sign.

(The root of the error in this case is that multiple threads are attempting
to read a single flow dump from a shared fd.  That should work, but the
kernel has an internal race that can result in one or more of those threads
hitting the EINVAL case at the start of netlink_dump().  The EINVAL is
harmless in this case and userspace should be able to ignore it, but
reporting the EINVAL as if it were a 22-byte message received in userspace
throws a real wrench in the works.)

This bug makes me think that there are probably not many programs doing
multithreaded Netlink dumps.  Maybe it is good that we are considering
other approaches.

VMware-BZ: #1255704
Reported-by: Mihir Gangar <gangarm@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>