shared/loop-util: use longer delay when waiting for loop device

The kernel may be syncing a file system or doing something else that requires
more time. So make the delay a bit longer, but provide some feedback and also
grow the delay exponentially (though with a long exponent). If the kernel is
doing something else, no need to repeat so often. With 38 attempts, we get a
total of slightly above 5000 ms.

I wrote this when I thought that the the delay is not long enough. It turned
out that we were blocking the file system on the loop device, so waiting longer
wasn't helpful. But I think it's nicer to do it this way anyway.

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