Always sync the global ZFS config cache to reflect the new mosconfig

This fixes out of date zpool.cache for root pools, which can cause issues
such as confusion of zdb etc.

MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276123 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f