restripe: make sure zero buffer is always large enough.

restripe: make sure zero buffer is always large enough.

If restripe is called to restore stripes of one size and then
save stripes with a larger chunk size, the 'zero' buffer will not
be large enough and a double-degraded RAID6 will over-run the buffer.

So record the current size of the zero buffer and use it when deciding
if we need to allocate a new buffer.

Reported-by: Brad Campbell <lists2009@fnarfbargle.com>
Signed-off-by: NeilBrown <neilb@suse.de>