Reduce number of places where global control_softc is used.

At some point we may want to have several CTL instances, and that is not
really impossible.

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275942 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f