/dev/null: add IORING_OP_URING_CMD support

This patch adds support for the io_uring command pass through, aka
IORING_OP_URING_CMD, to the /dev/null driver.  As with all of the
/dev/null functionality, the implementation is just a simple sink
where commands go to die, but it should be useful for developers who
need a simple IORING_OP_URING_CMD test device that doesn't require
any special hardware.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paul Moore <paul@paul-moore.com>