journald: when processing a native message, bail more quickly on overbig messages

We'd first parse all or most of the message, and only then consider if it
is not too large. Also, when encountering a single field over the limit,
we'd still process the preceding part of the message. Let's be stricter,
and check size limits early, and let's refuse the whole message if it fails
any of the size limits.

(cherry picked from commit 964ef920)
(cherry picked from commit 625d4f9d)