iomap: Switch to offset_in_page for clarity

Instead of open-coding pos & (PAGE_SIZE - 1) and pos & ~PAGE_MASK, use
the offset_in_page macro.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>