bpf, arm: start flushing icache range from header

During review I noticed that the icache range we're flushing should
start at header already and not at ctx.image.

Reason is that after 55309dd3

 ("net: bpf: arm: 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: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Nicolas Schichan <nschichan@freebox.fr>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>