Since commit d3f76110
newly allocated bvecs aren't initialised to NULL, so we have
to be more careful about freeing a bio which only managed
to get a few pages allocated to it.  Otherwise the resync
process crashes.

This patch is appropriate for 2.6.29-stable.

Cc: stable@kernel.org
Cc: "Jens Axboe" <jens.axboe@oracle.com>
Reported-by: NGabriele Tozzi <gabriele@tozzi.eu>
Signed-off-by: NNeilBrown <neilb@suse.de>