bpf: Convert BPF_DISPATCHER to use static_call() (not ftrace)

The dispatcher function is currently abusing the ftrace __fentry__
call location for its own purposes -- this obviously gives trouble
when the dispatcher and ftrace are both in use.

A previous solution tried using __attribute__((patchable_function_entry()))
which works, except it is GCC-8+ only, breaking the build on the
earlier still supported compilers. Instead use static_call() -- which
has its own annotations and does not conflict with ftrace -- to
rewrite the dispatch function.

By using: return static_call()(ctx, insni, bpf_func) you get a perfect
forwarding tail call as function body (iow a single jmp instruction).
By having the default static_call() target be bpf_dispatcher_nop_func()
it retains the default behaviour (an indirect call to the argument
function). Only once a dispatcher program is attached is the target
rewritten to directly call the JIT'ed image.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Tested-by: Björn Töpel <[email protected]>
Tested-by: Jiri Olsa <[email protected]>
Acked-by: Björn Töpel <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Link: https://lkml.kernel.org/r/Y1/oBlK0yFk5c/[email protected]
Link: https://lore.kernel.org/bpf/[email protected]

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