Fix 2 dc stream buffer

Fix 2 dc stream buffer

To meet requirements of Common Criteria certification vulnerablility
assessment. Static code analysis has been run and found the following
Error: DC.STREAM_BUFFER (CWE-120): [#def46]
mdadm-4.2: dont_call: "fscanf" assumes an arbitrarily
long string, so callers must use correct precision specifiers or
never use "fscanf".

The change is to define a value for string %s.

V2: Tighten the value in policy.c to match the limit of the metadata.
Add a change to policy_save_path() to use correct precision on the
fscanf call.

Signed-off-by: Nigel Croxon <ncroxon@redhat.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>