spi: Use struct_size() helper

The Documentation/process/deprecated.rst suggests to use flexible array
members to provide a way to declare having a dynamically sized set of
trailing elements in a structure.This makes code robust agains bunch of
the issues described in the documentation, main of which is about the
correctness of the sizeof() calculation for this data structure.

Due to above, prefer struct_size() over open-coded versions.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230714091748.89681-5-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -13,6 +13,7 @@
 #include <linux/gpio/consumer.h>
 #include <linux/kthread.h>
 #include <linux/mod_devicetable.h>
+#include <linux/overflow.h>
 #include <linux/scatterlist.h>
 #include <linux/slab.h>
 #include <linux/u64_stats_sync.h>
@@ -1085,6 +1086,8 @@ struct spi_transfer {
  * @state: for use by whichever driver currently owns the message
  * @resources: for resource management when the SPI message is processed
  * @prepared: spi_prepare_message was called for the this message
+ * @t: for use with spi_message_alloc() when message and transfers have
+ *	been allocated together
  *
  * A @spi_message is used to execute an atomic sequence of data transfers,
  * each represented by a struct spi_transfer.  The sequence is "atomic"
@@ -1139,6 +1142,9 @@ struct spi_message {
 
 	/* List of spi_res resources when the SPI message is processed */
 	struct list_head        resources;
+
+	/* For embedding transfers into the memory of the message */
+	struct spi_transfer	t[];
 };
 
 static inline void spi_message_init_no_memset(struct spi_message *m)
@@ -1199,16 +1205,13 @@ static inline struct spi_message *spi_message_alloc(unsigned ntrans, gfp_t flags
 {
 	struct spi_message *m;
 
-	m = kzalloc(sizeof(struct spi_message)
-			+ ntrans * sizeof(struct spi_transfer),
-			flags);
+	m = kzalloc(struct_size(m, t, ntrans), flags);
 	if (m) {
 		unsigned i;
-		struct spi_transfer *t = (struct spi_transfer *)(m + 1);
 
 		spi_message_init_no_memset(m);
-		for (i = 0; i < ntrans; i++, t++)
-			spi_message_add_tail(t, m);
+		for (i = 0; i < ntrans; i++)
+			spi_message_add_tail(&m->t[i], m);
 	}
 	return m;
 }