Fix: adapt to kernel relative references

Fix: adapt to kernel relative references

Upstream Linux commit 46e0c9be20 introduces relative references in the
struct tracepoint array of pointers.

Up to (including) v4.19-rc7, the upstream kernel has a type mismatch bug
that allows it to pass an out-of-bound end of array to modules
coming/going notifiers.

The fix for upstream Linux is to introduce a new type: tracepoint_ptr_t,
which can be used to adequately iterate on the array. It is introduced
prior to v4.19 as commit 9c0be3f6b5d77 "tracepoint: Fix tracepoint array
element size mismatch".

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