MFC r348558: MFV r348535:
9677 panic from zio_write_gang_block() when creating dump device on fragmented rpool

illumos/illumos-gate@7341a7de4f0489193e0cfe11049a7bcf1596a4db

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Brad Lewis <brad.lewis@delphix.com>