Fix a couple of things in the internal mbuf allocation interface:

• Make sure that m_mballoc() really doesn't allow over nmbufs mbufs to be allocated from mb_map. In the case where nmbufs-reserved space is not an exact multiple of PAGE_SIZE (which it should be, but anyway...), we hold nmbufs as an absolute maximum which need not ever be reached.

• Clean up m_clalloc(); make it more consistent in the sense that the first argument `ncl' really means "the number of clusters ensured to be allocated" and not "the number of pages worth of clusters to be allocated," as was previously the case. This also makes it consistent with m_mballoc() as well as the comment that preceeds it.

Reviewed by: jlemon