slob: clean up the code

- Use NULL instead of plain 0;
- Rename slob_page() to is_slob_page();
- Define slob_page() to convert void* to struct slob_page*;
- Rename slob_new_page() to slob_new_pages();
- Define slob_free_pages() accordingly.

Compile tests only.

Signed-off-by: WANG Cong <wangcong@zeuux.org>
Signed-off-by: Matt Mackall <mpm@selenic.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>