minor update

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
add wb->type guard in flush_plogs

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
explicitly check the bio dir

bio_data_dir(bio) == WRITE or READ is correct

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>