Implement lttng_iterate_fd wrapper

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fix: statedump: missing locking, use lttng_iterate_fd

- Take a spinlock around use of dentry->d_name.name.
- Use lttng_iterate_fd wrapper (and thus iterate_fd() from Linux kernels
  from v3.7 and onward).

Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
instrumentation: sync with Linux commit a1ce392

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