erofs-utils: lib: switch erofs_bread() to passing offset instead of block number

Source kernel commit: 469ad583c1293f5d9f45183050b3beeb4a8c3475

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20251224183131.2302377-8-hsiangkao@linux.alibaba.com