erofs: simplify z_erofs_read_fragment()

A trivial cleanup to make the fragment handling logic more clear.

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230817082813.81180-1-hsiangkao@linux.alibaba.com