nmbclusters on amd64

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

For many years on x86, I've been patching the kernel to set nmbclusters 
in the absence of it being alterable at run time. So here, it is set to 
32768:

# gdb /netbsd
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Reading symbols from /netbsd...(no debugging symbols found)...done.
(gdb) print nmbclusters
$1 = 32768
(gdb) quit

Yet according to sysctl it is 16384:
# sysctl kern.mbuf.nmbclusters
kern.mbuf.nmbclusters = 16384

and it's not alterable:
# sysctl -w kern.mbuf.nmbclusters=32768
sysctl: kern.mbuf.nmbclusters: Invalid argument

This is on:

# uname -rp
7.0_STABLE amd64

However, on the following:

# uname -rp
5.2_STABLE i386

all is as expected:

# sysctl kern.mbuf.nmbclusters
kern.mbuf.nmbclusters = 32768

So, how do I alter the number of nmbclusters on netbsd-7 amd64 to avoid 
being told to do something which I can't do?:

WARNING: mclpool limit reached; increase kern.mbuf.nmbclusters

--
Stephen