lru_cache: use correct type in sizeof for allocation

This has no actual effect, since sizeof(struct hlist_head) ==
sizeof(struct hlist_head *), but it's still the wrong type to use.

The semantic match that finds this problem:
// <smpl>
@@
type T;
identifier x;
@@
T *x;
...
* x = kzalloc(... * sizeof(T*) * ..., ...);
// </smpl>

[akpm@linux-foundation.org: use kcalloc()]
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Lars Ellenberg <lars@linbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>