erofs: fix encoded extents handling

 - The MSB 32 bits of `z_fragmentoff` are available only in extent
records of size >= 8B.

 - Use round_down() to calculate `lstart` as well as increase `pos`
   correspondingly for extent records of size == 8B.

Fixes: 1d191b4c ("erofs: implement encoded extent metadata")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250408114448.4040220-2-hsiangkao@linux.alibaba.com