bpf, arm64: start flushing icache range from header

While recently going over ARM64's BPF code, I noticed that the icache
range we're flushing should start at header already and not at ctx.image.

Reason is that after b569c1c6 ("net: bpf: arm64: address randomize
and write protect JIT code"), we also want to make sure to flush the
random-sized trap in front of the start of the actual program (analogous
to x86). No operational differences from user side.
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NZi Shen Lim <zlim.lnx@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/arm64/net/bpf_jit_comp.c

@@ -740,7 +740,7 @@ void bpf_int_jit_compile(struct bpf_prog *prog)
 	if (bpf_jit_enable > 1)
 		bpf_jit_dump(prog->len, image_size, 2, ctx.image);
 
-	bpf_flush_icache(ctx.image, ctx.image + ctx.idx);
+	bpf_flush_icache(header, ctx.image + ctx.idx);
 
 	set_memory_ro((unsigned long)header, header->pages);
 	prog->bpf_func = (void *)ctx.image;