default authentication needs to be at least ntlmv2 security for cifs mounts

We had planned to upgrade to ntlmv2 security a few releases ago,
and have been warning users in dmesg on mount about the impending
upgrade, but had to make a change (to use nltmssp with ntlmv2) due
to testing issues with some non-Windows, non-Samba servers.

The approach in this patch is simpler than earlier patches,
and changes the default authentication mechanism to ntlmv2
password hashes (encapsulated in ntlmssp) from ntlm (ntlm is
too weak for current use and ntlmv2 has been broadly
supported for many, many years).

Signed-off-by: Steve French <smfrench@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>