reduce sub-block DIO serialisation v2

[Christoph Hellwig <hch@xxxxxx>]

This takes the approach from Dave, but adds a new flag instead of abusing
the nowait one, and keeps a simpler calling convention for iomap_dio_rw.

Changes since v2:
 - rename the new flags
 - add an EOF check for subblock I/O
 - minor cleanups