Fix: race between statedump and library destructor

Fix: race between statedump and library destructor

The locking scheme for ust_lock() returns a teardown state (variable
lttng_ust_comm_should_quit) which is set by library destructor with lock
held.

It requires that when ust listener threads use this lock to protect
against concurrent accesses to a data structure, in addition to take
the lock, they need to check the return value of ust_lock() and
skip their critical section entirely if the return value indicates
that teardown is ongoing.

Iteration over all loaded libraries by lttng_ust_dl_update() starts by
iter_begin which grabs the lock, and sets data->cancel state
appropriately if teardown is ongoing. Then extract_bin_info_events()
uses the data->cancel state to skip over use of the protected structures
as needed, but iter_end() fails to take this data->cancel state into
account. Therefore, it can access data structures concurrently while
their teardown is ongoing which leads to crashes.

Fixes: #1169
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>