Previously, sysfs read() and write() calls looked for sysfs_ops in the struct 
sysfs_dir, in the kobject. Since objects belong to a subsystem, and is a member
of a group of like devices, the sysfs_ops have been moved to struct subsystem,
and are referenced from there.

The only remaining member of struct sysfs_dir is the dentry of the object's 
directory. That is moved out of the dir struct and directly into struct kobject.
That saves us 4 bytes/object.

All of the sysfs functions that referenced the struct have been changed to just
reference the dentry.