iomap: helper to trim pos/bytes to within folio

Several buffered write based iteration callbacks duplicate logic to
trim the current pos and length to within the current folio. Factor
this into a helper to make it easier to relocate closer to folio
lookup.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/20250506134118.911396-5-bfoster@redhat.com


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