iomap: avoid avoid truncating 64-bit offset to 32 bits

iomap: avoid avoid truncating 64-bit offset to 32 bits

on 32-bit kernels, iomap_write_delalloc_scan() was inadvertently using a
32-bit position due to folio_next_index() returning an unsigned long.
This could lead to an infinite loop when writing to an xfs filesystem.

Signed-off-by: Marco Nelissen <marco.nelissen@gmail.com>
Link: https://lore.kernel.org/r/20250109041253.2494374-1-marco.nelissen@gmail.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>