Rework of the kernel message buffer cache

[Justus Winter]

Hello,

I recently increased the maximum size of kernel message buffers that
are cached in commit 73314404.  This greatly improved the overall
system performance.

This is a rework of said cache mechanism.  It reduces the number of
cache misses even further, but its impact on performance is unclear to
me.  Please test this ;)

Otoh I believe this is the right thing to do even if it doesn't
improve the performance, as the current solution might be worse in a
multi-cpu setting, as the `ipc_kmsg_cache' would bounce back and forth
between per-cpu caches.  Also, it's cleaner :)

Thanks,
Justus