nvmet-tcp: peek icreq before starting TLS

Incoming connection might be either 'normal' NVMe-TCP connections
starting with icreq or TLS handshakes. To ensure that 'normal'
connections can still be handled we need to peek the first packet
and only start TLS handshake if it's not an icreq.
With that we can lift the restriction to always set TREQ to
'required' when TLS1.3 is enabled.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>