x86, mce: trivial clean up for mce-inject.c

Fix for:

WARNING: Use #include <linux/uaccess.h> instead of <asm/uaccess.h>
+#include <asm/uaccess.h>

WARNING: usage of NR_CPUS is often wrong - consider using cpu_possible(), num_possible_cpus(), for_each_possible_cpu(), etc
+       if (m.cpu >= NR_CPUS || !cpu_online(m.cpu))

ERROR: trailing whitespace
+/* $

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>