os/bluestore: Cleanup around bluestore_blob_t::get_ondisk_length()

Split get_ondisk_length() into
- get_ondisk_capacity()
- get_ondisk_length()
The change is done to amplify the difference between disk space used,
and potential capacity for disk space use.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>