MFV r337182: 9330 stack overflow when creating a deeply nested dataset

Datasets that are deeply nested (~100 levels) are impractical. We just put
a limit of 50 levels to newly created datasets. Existing datasets should
work without a problem.

illumos/illumos-gate@5ac95da7d61660aa299c287a39277cb0372be959

Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Author: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>