Add dm-userspace target

This patch adds the dm-userspace target to the kernel.  I
have tested
it successfully on both i386 and x86_64 SMP machines.

The idea of dm-userspace is to export an interface for
userspace
control over device-mapper block placement decisions.  It
effectively
allows device-mapper target logic to be implemented in
userspace.

I have a library that provides a simple record-based
userspace
interface for communication with a dm-userspace target,
available
here:

  http://static.danplanet.com/dm-userspace/libdmu-0.3.ta
r.gz

The library tarball also contains an example.c, which
demonstrates how
to write a userspace application (in about 16 lines of
actual code).

I would really like feedback about the kernel module.  I'm
definitely
willing to do whatever work is needed to make this an
acceptable part
of device-mapper.

-- 
Dan Smith
IBM Linux Technology Center
Open Hypervisor Team
email: danmsus.ibm.com
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