x86: cet: switch to run_in_user() to fix #DF exception

Commit: 8cd86535(x86: get rid of ring0stacktop) makes old test
application trigger #DF. To fix the issue, refactored the code
using run_in_user() which is adapted to the change well.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Message-Id: <20220406132446.32679-1-weijiang.yang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>