Bump the largest record we can cope with from 1k to 8k. Other users of
the hints file don't have any real limits, and longer records will
need to be written shortly.
Adjust the test of a KASSERT to better match the intent.

This assertion was added in r246213 as a guard against corrupted mbufs
arriving from drivers, the key distinguishing factor of said mbufs being
that they had a negative length. Given we're in a while loop specifically
designed to skip over zero-length mbufs, panicking on a zero-length mbuf
seems incorrect.

No objection from:	kib
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
Constify some static data.

MFC after:	2 weeks