Currently fileh_dirty_writeout() writes page via storeblk() in order -
- those with lower ->f_pgoffset are stored first.

This happens because current fileh_dirty_writeout() iterates whole
pagemap to find dirty pages and pagemap iteration is ordered by
f_pgoffset.

In upcoming patch we'll rework writeout code not to iterate through
whole pagemap, but only through dirty pages. However the property that
pages are emitted in canonical order is useful, so let's make sure via
tests this will stay preserved:

In mkdirty2() we modify pages in 2, 0 order, but the latter code checks
(via storeblk_trace()) they were actually stored in 0, 2 order.