x86/spectrev2: Don't set mode to SPECTRE_V2_NONE when retpoline is available.

x86/spectrev2: Don't set mode to SPECTRE_V2_NONE when retpoline is available.

When booting with noibrs we may accidentally set mode to
SPECTRE_V2_NONE, missing the fact that we will, in fact, be
using retpoline.

Change ibrs_select()'s behavior so that it only modifies
spectre_v2_mitigation mode if it was actually able to set
SPEC_CTRL_IBRS_INUSE flag.

Orabug: 28540376

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>