file: unify ->file_data and ->file_pos

file: unify ->file_data and ->file_pos

The only real use case of ->file_pos was by the sync engine to
avoid an lseek() if the offset was already correct. The e4defrag,
falloc, and fusion-aw engine also set ->file_pos, but that looks
like a case of copy-paste, since they don't actually use it.

Signed-off-by: Jens Axboe <axboe@kernel.dk>