OpenZFS 6876 - Stack corruption after importing a pool with a too-long name

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

Calling dsl_dataset_name on a dataset with a 256 byte buffer is asking
for trouble. We should check every dataset on import, using a 1024 byte
buffer and checking each time to see if the dataset's new name is longer
than 256 bytes.

OpenZFS-issue: https://www.illumos.org/issues/6876
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/ca8674e