SharedPtrRegistry: get_next must not delete while holding the lock

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>