scylla_io_setup: fix path join of default scylla directories
scylla_io_setup: fix path join of default scylla directories

Currently we are checking an invalid path of some default scylla directories,
the directories don't exist, so the tune will always be skipped. It caused by
two problem.

Problem 1: paths of default directories is invalid

Introduced by commit 5ec191536e, we try to tune some scylla default directories
if they exist. But the directory paths we try are wrong.

For example:
- What we check: /var/lib/scylla/commitlog_directory
- Correct one: /var/lib/scylla/commitlog

Problem 2: wrong path join

Introduced by commit 31ddb2145a, default_path might be replaced from
'/var/lib/scylla/' to '/var/lib/scylla'.

Our code tries to check an invalid path that is wrongly join, eg:
'/var/lib/scyllacommitlog'

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

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