m0. But m0 may be freed later, so trying to use sip6 at the end of this
function is wrong. My guess is that we want to reference the data area
of m (the mbuf about to be send) instead at this point.
Fix a panic on Xen (where a data area of a mbuf may be unmapped when the
mbuf is freed), and probably potential data/pool corruption in other cases.
b3b0d23068