x86, mce: use atomic_inc_return() instead of add by 1

Use atomic_inc_return() instead of atomic_add_return() by 1.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>