netlink: Fix handling of error from netlink_dump().

netlink_dump() returns a negative errno value on error.  Until now,
netlink_recvmsg() directly recorded that negative value in sk->sk_err, but
that's wrong since sk_err takes positive errno values.  (This manifests as
userspace receiving a positive return value from the recv() system call,
falsely indicating success.) This bug was introduced in the commit that
started checking the netlink_dump() return value, commit b44d211e (netlink:
handle errors from netlink_dump()).

Multithreaded Netlink dumps are one way to trigger this behavior in
practice, as described in the commit message for the userspace workaround
posted here:
    http://openvswitch.org/pipermail/dev/2014-June/042339.html



This commit also fixes the same bug in netlink_poll(), introduced in commit
cd1df525 (netlink: add flow control for memory mapped I/O).

Signed-off-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>