x86/intel/spectre_v4: Deprecate spec_store_bypass_disable=userspace

x86/intel/spectre_v4: Deprecate spec_store_bypass_disable=userspace

Enforcing userspace-only spectre_v4 mitigations cannot be done performantly
when retpoline mitigations for spectre_v2 are in force. To do so we would
need to write MSR_IA32_SPEC_CTRL when entering and leaving kernel (i.e. system
calls, interrupts, etc.) Since retpoline is the preferred method of spectre_v2
mitigations exactly because it avoids writing this extremely slow MSR, adding
these two writes for SSBD bit management will make using retpoline pointless.

While there may be some cases where running with speculative storage bypass
enabled in kernel only is better even in presense of the extra writes to
MSR_IA32_SPEC_CTRL we don't expect this to be the case in majority of cases.
Plus removing this mode makes code less unreadable.

Orabug: 28814570

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>