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>

MFC after:	2 weeks