dpif-netdev: Make internally thread-safe by introducing a global mutex.

dpif-netdev: Make internally thread-safe by introducing a global mutex.

This can be improved later but it is the simple thing to do for now.

I marked a couple of races with XXX.  I don't have a really good solution
for these, but I hope to find one.  They may be harmless in practice.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>