further writeback updates V2

[Christoph Hellwig <hch@xxxxxx>]

This series contains two patches from Dave (with a rebase and minor updates
from me) which he posted as RFC earlier to reduce the buffer_head dependency
in the writeback path, and one patch from me to make use of bios embedded
into the ioend and bio chaining to further simplify and optimize the writeback
path.

They pass xfstests on 4k and 1k file systems.

Changes since V1:
 - various minor refactoring based on the comments from Brian