writeback: avoid unnecessary determine_dirtyable_memory call

determine_dirtyable_memory() is a rather costly function since it
    need many atomic reads for gathering zone/global page state.  But when we
    use vm_dirty_bytes && dirty_background_bytes, we don't need that costly
    calculation.

    This patch eliminates such unnecessary overhead.

    NOTE : newly added if condition might add overhead in normal path.
           But it should be _really_ small because anyway we need the
           access both vm_dirty_bytes and dirty_background_bytes so it is
           likely to hit the cache.