MFC r269430: MFV r269426:

Double test device size for ztest(1).

Illumos issue:
    5039 ztest should default to larger device sizes
    Author: Matthew Ahrens <mahrens@delphix.com>
MFC r269431: MFV r269427:

In dnode_children_t, use C99's "[]" idiom for declaring the variable
sized array dnc_children at the end of the structure.

This prevents the compiler from mistakenly optimizing away accesses
beyond the array's defined size.

Illumos issue:
    5038 Remove "old-style" flexible array usage in ZFS.
    Author: Justin T. Gibbs <justing@spectralogic.com>