pipe: follow up cleanup to previous

pipe: follow up cleanup to previous

The commited patch was incomplete.

- add back missing goto retry, noted by jhb
- 'if (error)'  -> 'if (error != 0)'
- consistently do:

if (error != 0)
    break;
continue;

instead of:

if (error != 0)
    break;
else
    continue;

This adds some 'continue' uses which are not needed, but line up with the
rest of pipe_write.