Dropping mb_map

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
most architectures currently create a submap of the kernel map for use
by the mbuf cluster allocator. I would like to effectively kill that and
use the kmem_map instead. This would allow a dynamic upper limit only
affected by the VA limit of kmem_map.

This case shouldn't be hit too often as the clusters are already
allocated from a pool, so the locking difference shouldn't make a big
difference. Comments?

Joerg