Fix: get_wait_shm() ust mutex deadlock

Fix: get_wait_shm() ust mutex deadlock

The main issue is that get_wait_shm() bypass the fork() wrapper (with
lttng_ust_nest_count), which is responsible for holding the UST mutex
across fork(). Therefore, when exiting the context of the child process,
we execute the destructor, which try to grab the UST mutex, which might
be in pretty much any state.

Given that we don't want this process to try to register to
lttng-sessiond (because this is internal to lttng-ust), we might want to
let it skip the destructor execution. This would actually be the easiest
way out.

Reported-by: changz <zheng.chang@emc.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>