Handle interrupts in __ublksrv_ctrl_cmd

The syscall io_uring_enter may return EINTR if it is interrupted by a
delivery of a signal. Since __ublksrv_ctrl_cmd already added an SQE, it's
easier for ublksrv to handle the error instead of the caller.

For example, if ublksrv_ctrl_start_dev() returns -EINTR, then the caller
has to either manually wait for the CQE or might call
ublksrv_ctrl_start_dev() again, resulting in an erroneous second SQE.
(Which may have no effect, but handling the interrupt seems the most
correct)

lib/ublksrv_cmd.c

@@ -123,7 +123,9 @@ static int __ublksrv_ctrl_cmd(struct ublksrv_ctrl_dev *dev,
 		return ret;
 	}
 
-	ret = io_uring_wait_cqe(&dev->ring, &cqe);
+	do {
+		ret = io_uring_wait_cqe(&dev->ring, &cqe);
+	} while (ret == -EINTR);
 	if (ret < 0) {
 		fprintf(stderr, "wait cqe: %s\n", strerror(-ret));
 		return ret;