common: move SharedPtrRegistry test after t.join

The thread created to test SharedPtrRegistry race conditions updates a
value ( ptr ) that is tested by the main gtest thread but is not
protected by a lock. Instead of adding a lock, the main thread tests
the value after pthread_join() on the child thread.

http://tracker.ceph.com/issues/6130 fixes #6130

Signed-off-by: Loic Dachary <loic@dachary.org>
SharedPtrRegistry: get_next must not delete while holding the lock

    bool get_next(const K &key, pair<K, VPtr> *next)

may indirectly delete the object pointed by next->second when
doing :

    *next = make_pair(i->first, next_val);

and it will deadlock (EDEADLK) when

    void operator()(V *to_remove) {
      {
	Mutex::Locker l(parent->lock);

tries to acquire the lock because it is already held. The
Mutex::Locker is isolated in a block and the *next* parameter is set
outside of the block.

A test case demonstrating the problem is added to test_sharedptr_registry.cc

http://tracker.ceph.com/issues/6117 fixes #6117

Signed-off-by: Loic Dachary <loic@dachary.org>