iomap: improve argument passing to iomap_read_folio_sync

Pass the iomap_iter and derive the map inside iomap_read_folio_sync
instead of in the caller, and use the more descriptive srcmap name for
the source iomap.  Stop passing the offset into folio argument as it
can be derived from the folio and the file offset.  Rename the
variables for the offset into the file and the length to be more
descriptive and match the rest of the code.

Rename the function itself to iomap_read_folio_range to make the use
more clear.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250710133343.399917-13-hch@lst.de


Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>