nvme-tcp: allocate socket file

When using the TLS upcall we need to allocate a socket file such
that the userspace daemon is able to use the socket.

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