Simplify get_undo_list() by dropping the unnecessary cast, removing the
size variable, and switching to kzalloc() instead of a kmalloc() followed
by a memset().

This cleanup was split then modified from Jes Sorenson's Task Notifiers
patches.
Signed-off-by: NMatt Helsley <matthltc@us.ibm.com>
Cc: Jes Sorensen <jes@sgi.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>