[CIFS] mount option sec=none not displayed properly in /proc/mounts

When the user specifies "sec=none" in a cifs mount, we set
sec_type as unspecified (and set a flag and the username will be
null) rather than setting sectype as "none" so
cifs_show_security was not properly displaying it in
cifs /proc/mounts entries.

Signed-off-by: Steve French <steve.french@primarydata.com>
Reviewed-by: Jeff Layton <jlayton@poochiereds.net>