[PATCH 6/9] bpf: simplify callback inuse tracking to boolean flag

[Marat Khalili]

> If it is only on/off now make it a bool.

Changing it to bool may have performance impact on some architectures. 
Also, we'll have to make it int again when adding support for 
multi-threaded data planes.

You seem to be using this API more than me so I will leave the decision 
to you (and other reviewers), but please consider the points above first.