fs-writeback.c: unify some common code

The btrfs merge looks like hell, because it changes fs-writeback.c, and
the crazy code has this repeated "estimate number of dirty pages"
counting that involves three different helper functions.  And it's done
in two different places.

Just unify that whole calculation as a "get_nr_dirty_pages()" helper
function, and the merge result will look half-way decent.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>