xfstest progress with cifs mounts to Samba

[Steve French]

Now that we have better fixed up the configuration required, and with
current cifs for-next branch - the xfstest runs are working better now
(mount with actimeo=0 helped in a few tests that had been failing),
at least for the easier case of cifs mounts (with posix extensions) to
Samba 4.2-pre.

At least two tests generic/237 (setfacl error) and generic/258
(testing for negative seconds since epoch) are still failing though
and need more analysis.

And there are a few dozen tests that are not attempted that probably
could be gotten to work.