In the code that translates logical drive LBAs to physical
drive LBAs if we overflow the raid map disk data array we
will get the wrong answers.  We do not expect that to happen,
but best to be on the safe side and guard against it anyway.
Reviewed-by: NScott Teel <scott.teel@pmcs.com>
Signed-off-by: NDon Brace <don.brace@pmcs.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>