driver core: clean up the logic to determine which /sys/dev/ directory to use

When a dev_t is set in a struct device, an symlink in /sys/dev/ is
created for it either under /sys/dev/block/ or /sys/dev/char/ depending
on the device type.

The logic to determine this would trigger off of the class of the
object, and the kobj_type set in that location.  But it turns out that
this deep nesting isn't needed at all, as it's either a choice of block
or "everything else" which is a char device.  So make the logic a lot
more simple and obvious, and remove the incorrect comments in the code
that tried to document something that was not happening at all (it is
impossible to set class->dev_kobj to NULL as the class core prevented
that from happening.

This removes the only place that class->dev_kobj was being used, so
after this, it can be removed entirely.

Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230331093318.82288-4-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/base.h

@@ -209,6 +209,16 @@ int devtmpfs_init(void);
 static inline int devtmpfs_init(void) { return 0; }
 #endif
 
+#ifdef CONFIG_BLOCK
+extern struct class block_class;
+static inline bool is_blockdev(struct device *dev)
+{
+	return dev->class == &block_class;
+}
+#else
+static inline bool is_blockdev(struct device *dev) { return false; }
+#endif
+
 /* Device links support */
 int device_links_read_lock(void);
 void device_links_read_unlock(int idx);