Fix: sessiond: kernel error accounting fd still open when unloading modules

Fix: sessiond: kernel error accounting fd still open when unloading modules

Issue
=====
I noticed that some kernel modules were not unloading properly when
shutting down the sessiond. Namely those modules:
  lttng_counter_client_percpu_64_modular
  lttng_ring_buffer_event_notifier_client

The call to `modprobe -r` for those modules is failing because the
kernel error accounting fd has not been closed.

Fix
===
Only call `modprobe -r` after all kernel resources (file descriptors)
have been released.

Side note
=========
The `modprobe_remove_lttng()` is currently printing that the removal of a
_OPTIONAL module was a success even if it failed. I rework this function
to print a debug message in such situation. I used the
`modprobe_lttng()` as a template for this function.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibb51425ef3deedf6e982084ef879bc83ac823ba3