ipc/sem.c: replace kvmalloc/memset with kvzalloc and use struct_size

Use kvzalloc() instead of kvmalloc() and memset().

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

This code was detected with the help of Coccinelle.

Link: http://lkml.kernel.org/r/20190131214221.GA28930@embeddedor
Signed-off-by: Gustavo A. R. Silva <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Manfred Spraul <[email protected]>
Cc: Kees Cook <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

ipc/sem.c

@@ -488,18 +488,14 @@ static inline void sem_rmid(struct ipc_namespace *ns, struct sem_array *s)
 static struct sem_array *sem_alloc(size_t nsems)
 {
 	struct sem_array *sma;
-	size_t size;
 
 	if (nsems > (INT_MAX - sizeof(*sma)) / sizeof(sma->sems[0]))
 		return NULL;
 
-	size = sizeof(*sma) + nsems * sizeof(sma->sems[0]);
-	sma = kvmalloc(size, GFP_KERNEL);
+	sma = kvzalloc(struct_size(sma, sems, nsems), GFP_KERNEL);
 	if (unlikely(!sma))
 		return NULL;
 
-	memset(sma, 0, size);
-
 	return sma;
 }