Limit mbuma damage. Suddenly ALL allocations with M_WAITOK are subject
to failing -- that is, allocations via malloc(M_WAITOK) that are required
to never fail -- if WITNESS is not defined. While everyone should be
running WITNESS, in any case, zone "Mbuf" allocations are really the only
ones that should be screwed with by this hack.

This hack is crashing people, and would continue to do so with or without
WITNESS. Things shouldn't be allocating with M_WAITOK with locks held,
but it's not okay just to always remove M_WAITOK when !WITNESS.

Reported by: Bernd Walter <ticso@cicely5.cicely.de>