Fix don't zero_label when replace with spare

Fix don't zero_label when replace with spare

When replacing a disk with non-wholedisk spare, we shouldn't zero_label
it. The wholedisk case already skip it. In fact, zero_label function
will fail saying device busy because it's already opened exclusively,
but since there's no error checking, the replace command will succeed,
causing great confusion.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #6369