Fix: syscall tracing: missing trigger actions

Fix: syscall tracing: missing trigger actions

Observed issue
==============

There is an issue when associating multiple actions to a single system
call, where the first action is effectively added, but the following
actions are silently ignored.

Cause
=====

The is caused by event creation of the 2nd event to be skipped because
lttng_syscall_filter_enable returns "-EBUSY", because the bit is already
set in the bitmap.

Solution
========

Fixing this double-add-trigger scenario requires changes to how the
lttng_syscall_filter_enable -EBUSY errors are dealt with on event
creation, but the problem runs deeper.

Given that many events may be responsible for requiring a bit to be set
in the bitmap of the syscall filter, we cannot simply clear the bit
whenever one event is disabled

Otherwise, this will cause situations where all events for a given
system call are disabled (due to the syscall filter bit being cleared)
whenever one single event for that system call is disabled (e.g. 2
triggers on the same system call, but with different actions).

We need to keep track of all events associated with this bit in the
bitmap, and only clear the bit when the reference count reaches 0.
This fixes the double-add-trigger issue by enturing that no -EBUSY
is returned when creating the second event, and fixes the
double-add-trigger-then-remove-trigger scenario as well.

Known drawbacks
===============

None.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I3c1e966e9720a2e8f4438ae2c98d83ffe318e424