x86, bpf, jit: prevent speculative execution when JIT is enabled

x86, bpf, jit: prevent speculative execution when JIT is enabled

CVE-2017-5753 (Spectre v1 Intel)

When constant blinding is enabled (bpf_jit_harden = 1), this adds
an observable speculation barrier before emitting x86 jitted code
for the BPF_ALU(64)_OR_X and BPF_ALU_LHS_X
(for BPF_REG_AX register) eBPF instructions. This is needed in order
to prevent speculative execution on out of bounds BPF_MAP array
indexes when JIT is enabled. This way an arbitary kernel memory is
not exposed through side-channel attacks.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>