Remove a 'This is dumb' comment that has been incorrect for at least a
decade: m_pulldown() is willing to consider ordinary mbufs writable.
Retain another, related, and also outdated comment, but with a caveat
that it is partially stale. Do not, for now, address the problem that
it raises (that only EXT_CLUSTER external storage is considered
writable, regardless of the results of M_WRITABLE() on the mbuf).

MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division