netlink-socket: Do not make flow_dump block on netlink socket.

Commit 93295354 (netlink-socket: Simplify multithreaded dumping
to match Linux reality.) makes the call to recvmsg() block if no
messages are available.  This can cause revalidator threads hanging
for long time or even deadlock when main thread tries to stop the
revalidator threads.

This commit fixes the issue by enabling the MSG_DONTWAIT flag in
the call to recvmsg().

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>