iomap: set accurate iter->pos when reading folio ranges

iomap: set accurate iter->pos when reading folio ranges

Advance iter to the correct position before calling an IO helper to read
in a folio range. This allows the helper to reliably use iter->pos to
determine the starting offset for reading.

This will simplify the interface for reading in folio ranges when iomap
read/readahead supports caller-provided callbacks.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>