nvme-tcp: request secure channel concatenation

Add a fabrics option 'concat' to request secure channel concatenation as
specified the NVME Base Specification v2.1, section 8.3.4.3: Secure Channel
Concatenation.
When secure channel concatenation is enabled a 'generated PSK' is inserted
into the keyring such that it's available after reset.

Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>