iomap: rework iomap_write_begin() to return folio offset and length

iomap_write_begin() returns a folio based on current pos and
remaining length in the iter, and each caller then trims the
pos/length to the given folio. Clean this up a bit and let
iomap_write_begin() return the trimmed range along with the folio.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/20250506134118.911396-7-bfoster@redhat.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>