Base the calculation of maxmbufmem in part on kmem_map size
instead of kernel_map size to prevent kernel memory exhaustion
by mbufs and a subsequent panic on physical page allocation
failure.

On architectures without a direct map all mbuf memory (except
for jumbo mbufs larger than PAGE_SIZE) comes from kmem_map.
It is the limiting factor hence.

For architectures with a direct map using the size of kmem_map
is a good proxy of available kernel memory as well.  If it is
much smaller the mbuf limit may be sub-optimal but remains
reasonable, while avoiding panics under exhaustion.

The overall mbuf memory limit calculation may be reconsidered
again later, however due to the many different mbuf sizes and
different backing KVM maps it is a tricky subject.

Found by:	pho's new network stress test
Pointed out by:	alc (kmem_map instead of kernel_map)
Tested by:	pho