crimson/os/seastore: don't set INVALID extents to CLEAN when reading

crimson/os/seastore: don't set INVALID extents to CLEAN when reading
extents

CLEAN_PENDING extents may be invalidated before read completes:

1. transaction A retired an laddr, which lead to a RetirePlaceHolder in
   Cache
2. transaction B try to read that extent, and replace A's
   RetirePlaceHolder with it;
3. transaction A commits and invalidate that extent;
4. transaction B complete reading that extent;

In this case, we shouldn't set the extent's state to CLEAN

Signed-off-by: Xuehan Xu <xxhdx1985126@gmail.com>