distributed_loader: use proper directory for opening SSTable
distributed_loader: use proper directory for opening SSTable

Previous implementation assumes that each SSTable resides directly
in table::datadir directory, while what should actually be used
is directory path from SSTable descriptor.
This patch prevents a regression when adding staging sstables support
for upload/ dir.

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