Extract split lock mitigation into common file

This reduces duplication of the split lock mitigation path.
Additionally, it allows extending the functionality of procsysctl into
also getting the split lock mitigation state.

common/meson.build

@@ -3,6 +3,7 @@ common_sources = [
     'common-logging.c',
     'common-governors.c',
     'common-profile.c',
+    'common-splitlock.c',
     'common-external.c',
     'common-helpers.c',
     'common-gpu.c',