x86/uv/time: Use a flexible array in struct uv_rtc_timer_head

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The
older style of one-element or zero-length arrays should no longer be
used[2].

struct uv_rtc_timer_head contains a one-element array cpu[1]. Switch it
to a flexible array and use the struct_size() helper to calculate the
allocation size. Also, save some heap space in the process[3].

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays
[3] https://lore.kernel.org/lkml/20200518190114.GA7757@embeddedor/

 [ bp: Massage a bit. ]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Steve Wahl <steve.wahl@hpe.com>
Link: https://lkml.kernel.org/r/20201001145608.GA10204@embeddedor

arch/x86/platform/uv/uv_time.c

@@ -52,7 +52,7 @@ struct uv_rtc_timer_head {
 	struct {
 		int	lcpu;		/* systemwide logical cpu number */
 		u64	expires;	/* next timer expiration for this cpu */
-	} cpu[1];
+	} cpu[];
 };
 
 /*
@@ -148,9 +148,8 @@ static __init int uv_rtc_allocate_timers(void)
 		struct uv_rtc_timer_head *head = blade_info[bid];
 
 		if (!head) {
-			head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
-				(uv_blade_nr_possible_cpus(bid) *
-					2 * sizeof(u64)),
+			head = kmalloc_node(struct_size(head, cpu,
+				uv_blade_nr_possible_cpus(bid)),
 				GFP_KERNEL, nid);
 			if (!head) {
 				uv_rtc_deallocate_timers();