Fix division by zero, causing floating point exception in a drive progress command.
Fix division by zero, causing floating point exception in a drive progress
command.

It was possible to read a value of zero from a busy controller used
as the divisor to calculate the remaining rebuild time.

Reported by:	Pavel Udovenko <udovenko att nic.ru>
Discussed with:	jhb
Approved by:	kib (mentor)
MFC after:	1 week

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