ofproto-dpif-xlate: Avoid deadlock on multicast snooping recursion.

ofproto-dpif-xlate: Avoid deadlock on multicast snooping recursion.

Until now, OVS did multicast snooping outputs holding the read-lock on
the mcast_snooping object.  This could recurse via a patch port to try to
take the write-lock on the same object, which deadlocked.  This patch fixes
the problem, by releasing the read-lock before doing any outputs.

It would probably be better to use RCU for mcast_snooping.  That would be
a bigger patch and less suitable for backporting.

Reported-by: Sameh Elsharkawy
Reported-at: https://github.com/openvswitch/ovs-issues/issues/153
Signed-off-by: Ben Pfaff <blp@ovn.org>