decouple simple direct I/O reads from iomap_dio_rw v2

Hi all,

this series reworks how the iomap simple dio read fast path is called and
decouples it from the generic iomap_dio_rw and the iomap iter.  This is
to make the iomap_iter_next transition easier and cleaner.  It slots in
after the first patch from Joanne's

[PATCH v3 00/20] iomap: convert to in-iter iomap_next() model

series, and the first patch from that is included as the first patch
here unmodified as reference.  If the approach looks good to everyone,
I'd suggest to include it in the next spin of that series.

Changes since v1:
 - fix i_dio_count on synchronous errors
 - use read in the name also for the __-helper