x86/bugs: ssbd_ibrs_selected called prematurely

x86/bugs: ssbd_ibrs_selected called prematurely

spectre_v2_select_mitigation considers the SSB mitigation that's been
selected in its own decision; specifically, it calls ssbd_ibrs_selected
when deciding whether to use IBRS.  The problem is, the SSB mitigation
hasn't been chosen yet, so ssbd_ibrs_selected will always return the
same result, possibly the wrong one.

Address this by splitting SSB initialization into two phases.  The
first, ssb_select_mitigation, picks the SSB mitigation mode, getting far
enough for spectre_v2_select_mitigation's needs, and the second,
ssb_init, sets up the necessary SSB state based on the mode.  The second
phase is necessary because it relies on the outcome of
spectre_v2_select_mitigation.

Fixes: 648702f587df ("x86/bugs/IBRS: Disable SSB (RDS) if IBRS is selected for spectre_v2.")
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
(cherry picked from commit 86a51ae1ce42d9f76e09e3c87c20c3625c6a1a6c)

Orabug: 28788839

Signed-off-by: Anjali Kulkarni <anjali.k.kulkarni@oracle.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>