db: fix computation of live disk usage stat after compaction
db: fix computation of live disk usage stat after compaction

sstable::data_size() is used by rebuild_statistics() which only
returns uncompressed data size, and the function called by it
expects actual disk space used by all components.
Boot uses add_sstable() which correctly updates the stat with
sstable::bytes_on_disk(). That's what needs to be used by
r__s() too.

Fixes #1592

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <20170525210055.6391-1-1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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