Bug#399316: postinst breaks

[Santiago Garcia Mantinan]

> Hej,
> I would rather not try my luck with NMUing mdadm even with a safe
> looking change. It is a critical package.

In fact, I'm almost positive that the -e is the wanted stuff there, and not
a -n, so the fix should be replacing echo with /bin/echo, I have tested it
and it seems to work, but being mdadm a critical package... I'd rather not
NMU it unless it is necesary.

Regards...
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