ftrace: use quint32 for context counter

Some old platforms don't have atomic integer supports for 64-bit wise
integer.

As indicated in GPUVis:

	// Erase all knowledge of this ctx so it can be reused

We can reuse numbers after end_ctx, so with we should be fine with
32-bit integer.

Let's use a 32 bit integer for context.

(cherry picked from commit 5ba32847)