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

Callers are happier that way, especially since we no longer need to
play with splitting offset into block number and offset within block,
passing the former to erofs_bread(), then adding the latter...

erofs_bread() always reads entire pages, anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>