MFV r325605: 8713 Buffer overflow in dsl_dataset_name()

MFV r325605: 8713 Buffer overflow in dsl_dataset_name()

illumos/illumos-gate@f37ae9a714b97eca91c74c680c20c750c7cf5c02
https://github.com/illumos/illumos-gate/commit/f37ae9a714b97eca91c74c680c20c750c7cf5c02

https://www.illumos.org/issues/8713
  If we're creating a pool with version >= SPA_VERSION_DSL_SCRUB (v11) we need to
  account for additional space needed by the origin dataset which will also be
  snapshotted: "poolname"+"/"+"$ORIGIN"+"@"+"$ORIGIN".
  Enforce this limit in pool_namecheck().

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: loli10K <ezomori.nozomu@gmail.com>

MFC after:	1 week