[CIFS] Add warning message when broken server fails SetFSInfo call

A reasonably common NAS server returns an error on the SetFSInfo of
the Unix capabilities. Log a message for this alerting the user
that the server may have problems with the Unix extensions,
and telling them what they can do to workaround it.

Unfortunately the server does not return other clues
that we could easily use to turn the Unix Extension support
off automatically in this case (since they claim to support it).

Signed-off-by: Steve French <sfrench@us.ibm.com>