Fix powerpc compile warnings

mm/page-writeback.c: In function 'bdi_position_ratio':
mm/page-writeback.c:622:3: warning: comparison of distinct pointer types lacks a cast [enabled by default]
page-writeback.c:635:4: warning: comparison of distinct pointer types lacks a cast [enabled by default]

Also fix gcc "uninitialized var" warnings.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>