amd64: Fix two typos of loader tunables

[zlei]

Summary

To match the sysctl MIBs and security(7).

Fixes: 2dec2b4a34b4 amd64: flush L1 data cache on syscall return with an error
Fixes: 17edf152e556 Control for Special Register Buffer Data Sampling mitigation

MFC after: 1 day

[zlei]

I guess this one is also typo, but not quite sure.

static int pmap_allow_2m_x_ept;
SYSCTL_INT(_vm_pmap, OID_AUTO, allow_2m_x_ept, CTLFLAG_RWTUN | CTLFLAG_NOFETCH,
    &pmap_allow_2m_x_ept, 0,
    "Allow executable superpage mappings in EPT");
...
---TUNABLE_INT_FETCH("hw.allow_2m_x_ept", &pmap_allow_2m_x_ept);
+++TUNABLE_INT_FETCH("vm.pmap.allow_2m_x_ept", &pmap_allow_2m_x_ept);

[kib]

In D42249#963817, @zlei wrote:

I guess this one is also typo, but not quite sure.

static int pmap_allow_2m_x_ept;
SYSCTL_INT(_vm_pmap, OID_AUTO, allow_2m_x_ept, CTLFLAG_RWTUN | CTLFLAG_NOFETCH,
    &pmap_allow_2m_x_ept, 0,
    "Allow executable superpage mappings in EPT");
...
---TUNABLE_INT_FETCH("hw.allow_2m_x_ept", &pmap_allow_2m_x_ept);
+++TUNABLE_INT_FETCH("vm.pmap.allow_2m_x_ept", &pmap_allow_2m_x_ept);

This one is certainly inconsistency, I have no opinion which one (hw or vm.pmap) the tunable should be. The sysctl knob must be under vm.pmap of course.