libbe: bail out early if the zfs kmod isn't loaded

As noted in the comment, we already know the rest of libbe_init() will
fail because there's no pool imported. Avoid the side effect by
checking beforehand and bailing out early.

With this, freebsd-update(8) should no longer trigger a load of the zfs
kmod just because it runs bectl check.

Reviewed by: jwmaag_gmail.com, rew
Differential Revision: https://reviews.freebsd.org/D36188