x86/mce/dev-mcelog: Use struct_size() helper in kzalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200617211734.GA9636@embeddedor

arch/x86/kernel/cpu/mce/dev-mcelog.c

@@ -345,7 +345,7 @@ static __init int dev_mcelog_init_device(void)
 	int err;
 
 	mce_log_len = max(MCE_LOG_MIN_LEN, num_online_cpus());
-	mcelog = kzalloc(sizeof(*mcelog) + mce_log_len * sizeof(struct mce), GFP_KERNEL);
+	mcelog = kzalloc(struct_size(mcelog, entry, mce_log_len), GFP_KERNEL);
 	if (!mcelog)
 		return -ENOMEM;