Two bugs in there:

- The virt oversize check should use the current bio hardware back
  size and the next bio front size, not the same bio. Spotted by
  Neil Brown.

- The segment size check should add hw front sizes, not total bio
  sizes. Spotted by James Bottomley
Acked-by: NJames Bottomley <James.Bottomley@SteelEye.com>
Acked-by: NNeilBrown <neilb@suse.de>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>