SMB3: Fix SMB3.1.1 guest mounts to Samba

Workaround problem with Samba responses to SMB3.1.1
null user (guest) mounts.  The server doesn't set the
expected flag in the session setup response so we have
to do a similar check to what is done in smb3_validate_negotiate
where we also check if the user is a null user (but not sec=krb5
since username might not be passed in on mount for Kerberos case).

Note that the commit below tightened the conditions and forced signing
for the SMB2-TreeConnect commands as per MS-SMB2.
However, this should only apply to normal user sessions and not for
cases where there is no user (even if server forgets to set the flag
in the response) since we don't have anything useful to sign with.
This is especially important now that the more secure SMB3.1.1 protocol
is in the default dialect list.

An earlier patch ("cifs: allow guest mounts to work for smb3.11") fixed
the guest mounts to Windows.

    Fixes: 6188f28b ("Tree connect for SMB3.1.1 must be signed for non-encrypted shares")

Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>