Fix: common: index_allocator_get_index_count() returns size

The lttng_index_allocator_get_index_count() should return the number of
allocated indexes. Right now it returns the maximum number of indexes.

I witnessed the following message when running testing the event
notifier error counter feature:
  Warning: Destroying index allocator with 4096 slot indexes still in use

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibbcb3f128553df0a4118901ef85ae6fe17ebcecf