retpoline: move lock/unlock of spec_ctrl_mutex into init_scattered_cpuid_features()

retpoline: move lock/unlock of spec_ctrl_mutex into init_scattered_cpuid_features()

init_scattered_cpuid_features() changes spectre mitigation state.
Not all callers obtained spec_ctrl_mutex.  Move the lock/unlock of
spec_ctrl_mutex to init_scattered_cpuid_features() rather than have
the callers obtain it.  That also allows init_scattered_cpuid_features()
to serialize just the spectre state changes instead of the entire
function.

Orabug: 27625404
Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>