bpf: make function skip_callee static and return NULL rather than 0

Function skip_callee is local to the source and does not need to
be in global scope, so make it static. Also return NULL rather than 0.
Cleans up two sparse warnings:

symbol 'skip_callee' was not declared. Should it be static?
Using plain integer as NULL pointer

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>