XSM: adjust Kconfig names

XSM: adjust Kconfig names

Since the Kconfig option renaming was not backported, the new uses of
involved CONFIG_* settings should have been adopted to the existing
names in the XSA-295 series. Do this now, also changing XSM_SILO to just
SILO to better match its FLASK counterpart.

To avoid breaking the Kconfig menu structure also adjust XSM_POLICY's
dependency (as was also silently done on master during the renaming).

Signed-off-by: Jan Beulich <jbeulich@suse.com>