bpf: Add struct bpf_trampoline_ops object

bpf: Add struct bpf_trampoline_ops object

In following changes we will need to override ftrace direct attachment
behaviour. In order to do that we are adding struct bpf_trampoline_ops
object that defines callbacks for ftrace direct attachment:

   register_fentry
   unregister_fentry
   modify_fentry

The new struct bpf_trampoline_ops object is passed as an argument to
__bpf_trampoline_link/unlink_prog functions.

At the moment the default trampoline_ops is set to the current ftrace
direct attachment functions, so there's no functional change for the
current code.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260606123955.345967-6-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>