Fix warning messages when mounting to older servers

When mounting to older servers, such as Windows XP (or even Windows 7),
the limited error messages that can be passed back to user space can
get confusing since the default dialect has changed from SMB1 (CIFS) to
more secure SMB3 dialect. Log additional information when the user chooses
to use the default dialects and when the server does not support the
dialect requested.

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>