x86/entry: Have entry_ibpb() invalidate return predictions

entry_ibpb() should invalidate all indirect predictions, including return
target predictions. Not all IBPB implementations do this, in which case the
fallback is RSB filling.

Prevent SRSO-style hijacks of return predictions following IBPB, as the return
target predictor can be corrupted before the IBPB completes.

  [ bp: Massage. ]

Signed-off-by: Johannes Wikner <kwikner@ethz.ch>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>