ublk: don't return 0 in case of any failure

Commit 2d786e6 ("block: ublk: switch to ioctl command encoding")
starts to reset local variable of 'ret' as zero, then if any failure
happens when handling the three IO commands, 0 can be returned to ublk
server.

Fix it by returning -EINVAL in case of command handling failure.

Cc: Christoph Hellwig <hch@lst.de>
Fixes: 2d786e6 ("block: ublk: switch to ioctl command encoding")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230420091104.1092972-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

drivers/block/ublk_drv.c

@@ -1315,6 +1315,7 @@ static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 	if (ret)
 		goto out;
 
+	ret = -EINVAL;
 	switch (_IOC_NR(cmd_op)) {
 	case UBLK_IO_FETCH_REQ:
 		/* UBLK_IO_FETCH_REQ is only allowed before queue is setup */