mcelog, fix buffer overflow warning

The current code does (flags & MCE_OVERFLOW) in the buffer overflow check.
MCE_OVERFLOW is defined as 0 (the bit position) and the check is always
false.

This patch resolves the problem by correctly using the MCE_OVERFLOW as a
bit position.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>