smb: client: fix null auth

Commit abdb1742 removed code that clears ctx->username when sec=none, so attempting
to mount with '-o sec=none' now fails with -EACCES.  Fix it by adding that logic to the
parsing of the 'sec' option, as well as checking if the mount is using null auth before
setting the username when parsing the 'user' option.

Fixes: abdb1742

 ("cifs: get rid of mount options string parsing")
Cc: stable@vger.kernel.org
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>