The zone is used to provide KVA for mapping BIOs so that we can pass
buffers to VOP_READ and VOP_WRITE. Its use doesn't really make sense:
it is limited globally to nswbuf/10 allocations, so this might not work
well if a large number of vnode MDs are in use. Furthermore, all I/O to
a MD device is handled with a dedicated thread, so we might as well just
have one reserved block per softc.
I suspect we should go further and add VOPs which enable reading and
writing with unmapped buffers. But this step lets us get rid of some
unncessary pbuf pre-allocations, which is my current goal.