ceph: Move the place for EOLDSNAPC handle in ceph_aio_write to easily understand

Only for ceph_sync_write, the osd can return EOLDSNAPC.so move the
related codes after the call ceph_sync_write.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>