iomap: Fix possible overflow condition in iomap_write_delalloc_scan

In the Linux kernel, the following vulnerability has been resolved:

iomap: Fix possible overflow condition in iomap_write_delalloc_scan

folio_next_index() returns an unsigned long value which left shifted by PAGE_SHIFT could possibly cause an overflow on 32-bit system. Instead use folio_pos(folio) + folio_size(folio), which does this correctly.