bpf: Add explicit cast to 'void *' for __BPF_DISPATCHER_UPDATE()

When building with clang:

  kernel/bpf/dispatcher.c:126:33: error: pointer type mismatch ('void *' and 'unsigned int (*)(const void *, const struct bpf_insn *, bpf_func_t)' (aka 'unsigned int (*)(const void *, const struct bpf_insn *, unsigned int (*)(const void *, const struct bpf_insn *))')) [-Werror,-Wpointer-type-mismatch]
          __BPF_DISPATCHER_UPDATE(d, new ?: &bpf_dispatcher_nop_func);
                                     ~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~
  ./include/linux/bpf.h:1045:54: note: expanded from macro '__BPF_DISPATCHER_UPDATE'
          __static_call_update((_d)->sc_key, (_d)->sc_tramp, (_new))
                                                              ^~~~
  1 error generated.

The warning is pointing out that the type of new ('void *') and
&bpf_dispatcher_nop_func are not compatible, which could have side
effects coming out of a conditional operator due to promotion rules.

Add the explicit cast to 'void *' to make it clear that this is
expected, as __BPF_DISPATCHER_UPDATE() expands to a call to
__static_call_update(), which expects a 'void *' as its final argument.

Fixes: c86df29d11 ("bpf: Convert BPF_DISPATCHER to use static_call() (not ftrace)")
Link: https://github.com/ClangBuiltLinux/linux/issues/1755
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221107170711.42409-1-nathan@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/dispatcher.c

@@ -123,7 +123,7 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
 			return;
 	}
 
-	__BPF_DISPATCHER_UPDATE(d, new ?: &bpf_dispatcher_nop_func);
+	__BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func);
 
 	if (new)
 		d->image_off = noff;