tracing/lock: provide lock_acquired event support for dynamic size string

Now that we can support the dynamic sized string, make the lock tracing
able to use it, making it safe against modules removal and consuming
the right amount of memory needed for each lock name

Changes in v2:
adapt to the __ending_string() updates and the opening_string() removal.

[ Impact: protect lock tracer against module removal ]

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>