netdump: send key before dump, in case dump fails

Previously, if an encrypted netdump failed, such as due to a timeout or
network failure, the key was not saved, so a partial dump was
completely useless.

Send the key first, so the partial dump can be decrypted, because even a
partial dump can be useful.

Reviewed by: bdrewery, markj
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D31453