x86/bugs: Enable IBPB flush on BPF JIT allocation

x86/bugs: Enable IBPB flush on BPF JIT allocation

Enable hardening against JIT spraying when Spectre-v2 mitigations are in
use. Specifically, issue an IBPB flush on BPF JIT memory reuse. Skip
enabling the IBPB flush if the BPF dispatcher is already using a retpoline
sequence.

This hardening applies only when BPF-JIT is in use. Guard the enabling
under CONFIG_BPF_JIT so that bugs.c still builds with CONFIG_BPF_JIT=n.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>