virt: sev-guest: Mark driver struct with __refdata to prevent section mismatch

As described in the added code comment, a reference to .exit.text is ok for
drivers registered via module_platform_driver_probe(). Make this explicit to
prevent the following section mismatch warning:

  WARNING: modpost: drivers/virt/coco/sev-guest/sev-guest: section mismatch in reference: \
    sev_guest_driver+0x10 (section: .data) -> sev_guest_remove (section: .exit.text)

that triggers on an allmodconfig W=1 build.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/4a81b0e87728a58904283e2d1f18f73abc69c2a1.1711748999.git.u.kleine-koenig@pengutronix.de

Author: Uwe Kleine-König

drivers/virt/coco/sev-guest/sev-guest.c

@@ -1203,8 +1203,13 @@ static void __exit sev_guest_remove(struct platform_device *pdev)
  * This driver is meant to be a common SEV guest interface driver and to
  * support any SEV guest API. As such, even though it has been introduced
  * with the SEV-SNP support, it is named "sev-guest".
+ *
+ * sev_guest_remove() lives in .exit.text. For drivers registered via
+ * module_platform_driver_probe() this is ok because they cannot get unbound
+ * at runtime. So mark the driver struct with __refdata to prevent modpost
+ * triggering a section mismatch warning.
  */
-static struct platform_driver sev_guest_driver = {
+static struct platform_driver sev_guest_driver __refdata = {
 	.remove_new	= __exit_p(sev_guest_remove),
 	.driver		= {
 		.name = "sev-guest",