smbd: Cache dfree information based on query path

smbd: Cache dfree information based on query path

Sub directories in a SMB share can have different free space information
(e.g. when a different file system is mounted there). Caching the dfree
information per SMB share will return invalid data. Address this by
switching to memcache and store the cached data based on the query path.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13446

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>