selinux: implement the security_uring_cmd() LSM hook

Add a SELinux access control for the iouring IORING_OP_URING_CMD
command.  This includes the addition of a new permission in the
existing "io_uring" object class: "cmd".  The subject of the new
permission check is the domain of the process requesting access, the
object is the open file which points to the device/file that is the
target of the IORING_OP_URING_CMD operation.  A sample policy rule
is shown below:

  allow <domain> <file>:io_uring { cmd };

Cc: stable@vger.kernel.org
Fixes: ee692a21

 ("fs,io_uring: add infrastructure for uring-cmd")
Signed-off-by: Paul Moore <paul@paul-moore.com>