s3:smb2_server: use the global signing key to check if signing is required

s3:smb2_server: use the global signing key to check if signing is required

If we have a channel session key, we also always have a global session key.

For multi-channel it's possible that the channel session key is not in place
yet, in that case the global session key needs to be used.

In both cases (reauth or session bind) we session setup requests need to be
signed.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>