snapshots: always create the snapshot directory
snapshots: always create the snapshot directory

We currently don't always create the snapshot directory as an
optimization. We have a test at sync time handling this use case.

This works well when all SSTables are created in the same directory, but
if we have more than one data directory than it may not work if we don't
have SSTables in all data directories.

We can fix it by unconditionally creating the directory.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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