erofs: refine erofs_iomap_begin()

 - Avoid calling erofs_map_dev() for unmapped extents;

 - Assign `iomap->addr` for inline extents too (since they have physical
   location).

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250716092254.3826715-1-hsiangkao@linux.alibaba.com