osd: fix wrong promotion after writing snapshot

osd: fix wrong promotion after writing snapshot

Let's assume the following scenario.
snap_create() -> write() -> tier_promote() -> read().
write() trigger to create a snapshot with current chunk_map, but
it does not update chunk_map even if the write() overwrites the region.
So, tier_promote() will overwrite the region by writing old chunk to there.

To avoid this, update_chunk_map_by_dirty is added by splitting
dec_refcount_by_dirty

Signed-off-by: Myoungwon Oh <myoungwon.oh@samsung.com>