Fix: statedump: check whether "files" is NULL, RCU semantic fix

We need to check if p->files is NULL before passing it to
files_fdtable(). Moreover, since the fdt is now protected by RCU, we
have to assume it can change between the read from
lttng_enumerate_task_fd() and the internal in-kernel read in
iterate_fd(). Therefore, move this rcu dereference into
lttng_dump_one_fd(), and perform the appropriate checks on max fds.

Fixes #799

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