sstable: create_links: capture dir by reference
sstable: create_links: capture dir by reference

Now that it's held with `do_with`.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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