Refactoring: context callbacks

Refactor the context callbacks so they take a private pointer as
argument.

Note that the callstack context internally uses a per-cpu stack across
context size calculation and recording to the ring buffer, which
requires that preemption is disabled across those operations.

The get_size_arg and _get_size callbacks are merged into a single
callback.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I7c4146481d35cb9ddef19f2cb5d61e56ebe142be