spi: Don't use flexible array in struct spi_message definition

The struct spi_message can be embedded into another structures.
With that the flexible array might be problematic as sparse
complains about it, although there is no real issue in the code
because when the message is embedded it doesn't use flexible array
member. That memeber is a private to spi_message_alloc() API, so
move it to that API in a form of an inherited data type.

Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Fixes: 75e308f ("spi: Use struct_size() helper"))
Closes: https://lore.kernel.org/r/20231009-onshore-underage-c58415adfd92-mkl@pengutronix.de
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20231010163100.89734-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: andy-shev

include/linux/spi/spi.h

@@ -1086,8 +1086,6 @@ 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"
@@ -1142,9 +1140,6 @@ 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)
@@ -1203,17 +1198,21 @@ struct spi_transfer *xfers, unsigned int num_xfers)
  */
 static inline struct spi_message *spi_message_alloc(unsigned ntrans, gfp_t flags)
 {
-	struct spi_message *m;
+	struct spi_message_with_transfers {
+		struct spi_message m;
+		struct spi_transfer t[];
+	} *mwt;
+	unsigned i;
+
+	mwt = kzalloc(struct_size(mwt, t, ntrans), flags);
+	if (!mwt)
+		return NULL;
 
-	m = kzalloc(struct_size(m, t, ntrans), flags);
-	if (m) {
-		unsigned i;
+	spi_message_init_no_memset(&mwt->m);
+	for (i = 0; i < ntrans; i++)
+		spi_message_add_tail(&mwt->t[i], &mwt->m);
 
-		spi_message_init_no_memset(m);
-		for (i = 0; i < ntrans; i++)
-			spi_message_add_tail(&m->t[i], m);
-	}
-	return m;
+	return &mwt->m;
 }
 
 static inline void spi_message_free(struct spi_message *m)