dumpon: fix encrypted dumps after commit 372557d8c3d

That commit moved key generation into a child process, including
a memory allocation referenced by a structure. The child wrote
the structure to the parent over a pipe, but did not write the
referenced allocation. The parent read the structure from the
child and used its pointer, which was bogus in the parent.

In the child, send both chunks of data to the parent. In the
parent, make a corresponding allocation and read both chunks.

Fixes: 372557d8c3d37dd0c1d9be56513a436393963848
Reviewed by: bdrewery, markj
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D31452