linuxkpi: Fix __ATTR_RO() in <linux/sysfs.h>

[Why]
The passed structure may not have a `.store` field. This is the case in
the amdgpu DRM driver starting with Linux 5.18.

Reviewed by:	manu
Approved by:	manu
Differential Revision:	https://reviews.freebsd.org/D43023

sys/compat/linuxkpi/common/include/linux/sysfs.h

@@ -54,7 +54,10 @@ struct attribute_group {
 	.attr = { .name = __stringify(_name), .mode = _mode },		\
 	.show = _show, .store  = _store,				\
 }
-#define	__ATTR_RO(_name)	__ATTR(_name, 0444, _name##_show, NULL)
+#define	__ATTR_RO(_name) {						\
+	.attr = { .name = __stringify(_name), .mode = 0444 },		\
+	.show = _name##_show,						\
+}
 #define	__ATTR_WO(_name)	__ATTR(_name, 0200, NULL, _name##_store)
 #define	__ATTR_RW(_name)	__ATTR(_name, 0644, _name##_show, _name##_store)
 #define	__ATTR_NULL	{ .attr = { .name = NULL } }