mcelog: Fix missing handling for flag MCE_GETCLEAR_FLAGS

This flag is used to handle some flags sent from kernel, like
overflow flag. But mcelog forgets to check and clean it once
it is set.

When accessing this flag via ioctl, I ignore failed return
value intentionally to avoid it reported over and over again.

Signed-off-by: Chen, Gong <gong.chen@linux.intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>