spi: axi-spi-engine: Start the program length at zero

spi_engine_optimize_message() allocates the program with kzalloc_flex(), which sets the __counted_by() counter "length" to the allocated size when the compiler supports __builtin_counted_by_ref(). The program is then filled by spi_engine_program_add_cmd(), which increments "length" and stores at length - 1, expecting it to start at zero as it did when the program came from kzalloc().

Reset "length" after the allocation, since it is a "usage" counter, not a "total allocation" counter.

Build tested ARCH=x86_64 defconfig with CONFIG_SPI_AXI_SPI_ENGINE=m and W=1, with GCC 16.2.0.

---
 drivers/spi/spi-axi-spi-engine.c | 2 ++
 1 file changed, 2 insertions(+)

index ead5b295e680..9cfee409f15c 100644
@@ -818,6 +818,8 @@ static int spi_engine_optimize_message(struct spi_message *msg)
 	p = kzalloc_flex(*p, instructions, p_dry.length + 1);
 	if (!p)
 		return -ENOMEM;
+	/* spi_engine_program_add_cmd() counts the instructions as it adds them. */
+	p->length = 0;
 
 	spi_engine_compile_message(msg, false, p);
 

2.55.0