smb3.1.1: enable negotiating stronger encryption by default

Now that stronger encryption (gcm256) has been more broadly
tested, and confirmed to work with multiple servers (Windows
and Azure for example), enable it by default.  Although gcm256 is
the second choice we offer (after gcm128 which should be faster),
this change allows mounts to server which are configured to
require the strongest encryption to work (without changing a module
load parameter).

Signed-off-by: Steve French <stfrench@microsoft.com>
Suggested-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>