libbe: recursively promote deep BE datasets

beadm will recursively promote deep BE datasets. In order to match the
beadm behavior, we need to recursively iterate over child filesystems
and promote them along the way.

This patch further refines the work from D40903, completing the fix for
promotion.

Reviewed by: kevans, rew
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D40972