KVM: SVM: let alternatives handle the cases when RSB filling is required

Remove superfluous RSB filling after a VMEXIT when the CPU already has
flushed the RSB after a VMEXIT when AutoIBRS is enabled.

The initial implementation for adding RETPOLINES added an ALTERNATIVES
implementation for filling the RSB after a VMEXIT in commit 117cc7a9
("x86/retpoline: Fill return stack buffer on vmexit").

Later, X86_FEATURE_RSB_VMEXIT was added in commit 9756bba2
("x86/speculation: Fill RSB on vmexit for IBRS") to handle stuffing the
RSB if RETPOLINE=y *or* KERNEL_IBRS=y, i.e. to also stuff the RSB if the
kernel is configured to do IBRS mitigations on entry/exit.

The AutoIBRS (on AMD) feature implementation added in commit e7862eda
("x86/cpu: Support AMD Automatic IBRS") used the already-implemented logic
for EIBRS in spectre_v2_determine_rsb_fill_type_on_vmexit() -- but did not
update the code at VMEXIT to act on the mode selected in that function --
resulting in VMEXITs continuing to clear the RSB when RETPOLINES are
enabled, despite the presence of AutoIBRS.

Signed-off-by: Amit Shah <amit.shah@amd.com>
Link: https://lore.kernel.org/r/20240807123531.69677-1-amit@kernel.org


[sean: massage changeloge, drop comment about AMD not needing RSB_VMEXIT_LITE]
Signed-off-by: Sean Christopherson <seanjc@google.com>