Fix: perform the initialization memory barrier out of loop body

Fix: perform the initialization memory barrier out of loop body

The memory barrier used by the client thread should be performed
after the lttng_sessiond_ready counter has been seen to have
reached zero.

This ensures that loads are not speculatively performed before
this point as the thread will interact with data structures
initialized by the support threads for which it was waiting for
the initialization to complete.

See the comment as to why this read barrier is promoted to a
full barrier.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>