nvme-tcp: fix selinux denied when calling sock_sendmsg

In a SELinux enabled kernel, socket_create() initializes the security
label of the socket using the security label of the calling process,
this typically works well.

However, in a containerized environment like Kubernetes, problem arises
when a privileged container(domain spc_t) connects to an NVMe target and
mounts the NVMe as persistent storage for unprivileged containers(domain
container_t).

This is because the container_t domain cannot access resources labeled
with spc_t, resulting in socket_sendmsg returning -EACCES.

The solution is to use socket_create_kern() instead of socket_create(),
which labels the socket context to kernel_t.  Access control will then
be handled by the VFS layer rather than the socket itself.

Signed-off-by: Peijie Shao <shaopeijie@cestc.cn>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>