OpenZFS 9677 - panic from zio_write_gang_block()

Panic from zio_write_gang_block() when creating dump device
on fragmented rpool.

Authored by: Brad Lewis <brad.lewis@delphix.com>
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>
Ported-by: Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://illumos.org/issues/9677
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/7341a7d
Closes #7975