further bpf_existence_test code simplifications/clarifications

(mainly driven by the desire to make it clear this
is about *current* mainline state and not at-T-launch state
of things)

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I928f91704f78205ffe44611a3d3abe383c4e560b