ofproto-dpif-monitor: Fix deadlock.

Commit 6b59b543 (ovs-thread: Use fair (but nonrecursive)
rwlocks on glibc.) changed the rwlocks to nonrecursive,
writer-biased lock.  It also made the following deadlock
possible.

Assume BFD is used on both end of a link.  Consider the
following events:
1. Handler at one end received the BFD control packet with
   POLL flag set while holding the read lock of 'xlate_rwlock'.
   Since a BFD control packet with FINAL flag set should be
   sent back immediately, it calls the
   ofproto_dpif_monitor_port_send_soon(), in which, it tries
   to grab the 'monitor_mutex'.
2. The main thread needs to configure the ofproto-dpif-xlate
   module.  It tries to grab the write lock of 'xlate_rwlock'
   and is blocked by event 1.
3. The monitor thread, after acquired the 'monitor_mutex',
   wants to acquire the read lock of 'xlate_rwlock'.

Since the rwlock is now writer-biased, the attempt of acquiring
read lock in event 3 will be blocked by event 2.  This will
subsequently cause the block of event 1, since monitor thread
is holding the 'monitor_mutex'.  So the deadlock happens.

This commit resolves the above issue by removing the requirement of
acquiring 'monitor_mutex' in ofproto_dpif_monitor_port_send_soon().

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