osd: clear data digest when write_trunc

osd: clear data digest when write_trunc

like CEPH_OSD_OP_TRUNCATE, the truncate within CEPH_OSD_OP_WRITE should
clear oi.data_digest too.

consider such circumstance:
[write 0~10]
[write 5~10 [1@5]]

the first write will generate data_digest, if the second write don't
clear digest, it will continue calc digest and make crc mismatch.

Fixes: https://tracker.ceph.com/issues/50763
Signed-off-by: Zengran Zhang <zhangzengran@sangfor.com.cn>