kref: remove kref_set

Of the three uses of kref_set in the kernel:

 One really should be kref_put as the code is letting go of a
    reference,
 Two really should be kref_init because the kref is being
    initialised.

This suggests that making kref_set available encourages bad code.
So fix the three uses and remove kref_set completely.

Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>