ctf: force dtrace_ctf.ko to be loaded whenever dtrace.ko is

ctf: force dtrace_ctf.ko to be loaded whenever dtrace.ko is

DTrace userspace makes the simplifying assumption that dtrace_ctf.ko (containing
the CTF for the kernel, for built-in modules, and for shared types, but no code)
is always loaded whenever DTrace is usable.  (The CTF itself is in a non-loaded
section, but having dtrace_ctf.ko in the list of loaded modules means that we
can eliminate an annoying set of dtrace_ctf-related special case.)

We do this by introducing a dummy function dtrace_ctf_forceload() into the
dtrace_ctf module, which DTrace then calls: depmod will then arrange for
dtrace_ctf to be loaded when we need it.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>