ofproto-dpif: Don't hold mac rwlock while calling send_packet().

Holding the mac learning rwlock while calling send_packet() which
calls xlate_actions() is dangerous because somewhere deep in the chain
the same lock might be acquired causing a deadlock.  Though we haven't
run into this problem in testing, it seems prudent to refactor it.

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