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@embeddedorSigned-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

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;
 }