Mutex while updating statedump_pending is not needed

Take care of coverity warning ** CID 1226392:  Data race condition  (MISSING_LOCK)
/liblttng-ust/lttng-ust-comm.c: 1704 in lttng_ust_sockinfo_session_enabled()

It's not that the lock is missing, it's that we don't need to hold the
lock when doing the other modification.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>