bpf, arm64: start flushing icache range from header

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 b569c1c622c5 ("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: Daniel Borkmann <[email protected]>
Acked-by: Zi Shen Lim <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>