Fix: sessiond: return _OK on _SET_SESSION_SHM_PATH command success

Fix: sessiond: return _OK on _SET_SESSION_SHM_PATH command success

Issue
=====
When running the `test_crash` tests, I witness the following warning:
  Warning: Command returned an invalid status code, returning unknown error: command type = 40, ret = 0

The `cmd_set_session_shm_path()` function is returning 0 on success
instead of the expected LTTNG_OK which leads to this warning being
printed.

Fix
===
Return LTTNG_OK.

Side note
=========
I added the string version of the command name in the warning printing
for easier debugging.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I79fe41afd49a1c701e760e415b72aabdc9b25f6c