Trouble with recent auto-tuning changes

I ran into a panic while attempting to un-tar a large file on a
DreamPlug (arm-based system) running -current.  The source and dest of
the un-tar is the root filesystem on sdcard, and I get this:

panic: kmem_malloc(4096): kmem_map too small: 12582912 total allocated

Just before the panic I see the tar process get hung in a "nokva" wait.
12582912 is the value of VM_KMEM_SIZE from arm/include/vmparam.h.

In r245575 the init order for mbuf limits was changed from
SI_SUB_TUNABLES to SI_SUB_KMEM so that mbuf limits could be based on the
results of sizing kernel memory.  Unfortunately, the process of sizing
kernel memory relies on the mbuf limits; in kmeminit():

	vm_kmem_size = VM_KMEM_SIZE + nmbclusters * PAGE_SIZE;

Since r245575, nmbclusters is zero when this line of code runs.  If I
manually plugin "32768" (the number tunable_mbinit() comes up with for
this platform) in that line, the panic stops happening.

So we've got two problems here... one is the circular dependency in
calculating the mbuf limits.  The other is the fact that some
non-trivial amount of kernel memory we're allowing for mbufs is actually
being used for other things.  That is, if my system was actually using
all the mbufs that tunable_mbinit() allowed for, then this panic while
untarring a huge file would still have happened.

I arrive at the latter conclusion based on the fact that this panic
happens even if no network interfaces (other than lo0) are configured.
That is, nmbclusters == 0 is a reasonable approximation of my need for
network mbufs.  So something in the system needs to be taken into
account when sizing kernel memory to allow for whatever it is about
untarring a huge file that eats kernel memory (buffer cache?).

I can easily reproduce this if you need me to gather any specific info.

-- Ian