Changing return value of kprobe's version of bpf_get_func_ip
to return zero if the attach address is not on the function's
entry point.

For kprobes attached in the middle of the function we can't easily
get to the function address especially now with the CONFIG_X86_KERNEL_IBT
support.

If user cares about current IP for kprobes attached within the
function body, they can get it with PT_REGS_IP(ctx).
Suggested-by: NAndrii Nakryiko <andrii@kernel.org>
Acked-by: NAndrii Nakryiko <andrii@kernel.org>
Acked-by: NMartynas Pumputis <m@lambda.lt>
Signed-off-by: NJiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220926153340.1621984-6-jolsa@kernel.orgSigned-off-by: NAlexei Starovoitov <ast@kernel.org>