sstable_directory: fix off-by-one when calculating number of jobs
sstable_directory: fix off-by-one when calculating number of jobs

Number of jobs can be off-by-one if it's divisible by max threshold
(max_sstables_per_job), which results in one extra unneeded resharding
job.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <20200609163430.14155-1-1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla