Objecter: failed assert(tick_event==NULL) at osdc/Objecter.cc

Objecter: failed assert(tick_event==NULL) at osdc/Objecter.cc

When the Objecter timer erases the tick_event from its events queue and
calls tick() to dispatch it, if the Objecter::rwlock is held by shutdown(),
it waits there to get the rwlock. However, inside the shutdown function,
it checks the tick_event and tries to cancel it. The cancel_event function
returns false since tick_event is already removed from the events queue. Thus
tick_event is not set to NULL in shutdown(). Later the tick function return
ealier and doesn't set tick_event to NULL as well. This leads to the assertion
failure.

This is a regression introduced by an incorrect conflict resolution when
d790833 was backported.

Fixes: #11183
Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>