x86/IBRS: Don't try to change IBRS mode if IBRS is not available

x86/IBRS: Don't try to change IBRS mode if IBRS is not available

sysctl_ibrs_enabled is set properly when IBRS support is discovered
in init_scattered_cpuid_features(). We should simply return an error
when attempt is made to change IBRS mode when the feature is not supoorted.

There is also no need to call set_ibrs_inuse() when changing mode to 1:
this is already done by clear_ibrs_disabled().

Orabug: 27448280

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>