driver core: Remove completion from struct klist_node

Removing the completion from klist_node reduces its size from 64 bytes
to 28 on x86-64.  To maintain the semantics of klist_remove(), we add
a single list of klist nodes which are pending deletion and scan them.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>