MFC r305194: MFV r302642:

MFC r305194: MFV r302642:
6876 Stack corruption after importing a pool with a too-long name

illumos/illumos-gate@c971037baa5d64dfecf6d87ed602fc3116ebec41
https://github.com/illumos/illumos-gate/commit/c971037baa5d64dfecf6d87ed602fc3116ebec41

https://www.illumos.org/issues/6876
  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.

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>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Paul Dagnelie <pcd@delphix.com>