platform/x86: dell_rbu: Fix lock context warning

Fix a sparse lock context warning.

Signed-off-by: Stuart Hayes <stuart.w.hayes@gmail.com>
Link: https://lore.kernel.org/r/20250609184659.7210-2-stuart.w.hayes@gmail.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Author: stuartwhayes

drivers/platform/x86/dell/dell_rbu.c

@@ -91,7 +91,7 @@ static void init_packet_head(void)
 	rbu_data.imagesize = 0;
 }
 
-static int create_packet(void *data, size_t length)
+static int create_packet(void *data, size_t length) __must_hold(&rbu_data.lock)
 {
 	struct packet_data *newpacket;
 	int ordernum = 0;