/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>

drivers/char/mem.c

@@ -480,6 +480,11 @@ static ssize_t splice_write_null(struct pipe_inode_info *pipe, struct file *out,
 	return splice_from_pipe(pipe, out, ppos, len, flags, pipe_to_null);
 }
 
+static int uring_cmd_null(struct io_uring_cmd *ioucmd, unsigned int issue_flags)
+{
+	return 0;
+}
+
 static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
 {
 	size_t written = 0;
@@ -663,6 +668,7 @@ static const struct file_operations null_fops = {
 	.read_iter	= read_iter_null,
 	.write_iter	= write_iter_null,
 	.splice_write	= splice_write_null,
+	.uring_cmd	= uring_cmd_null,
 };
 
 static const struct file_operations __maybe_unused port_fops = {