Merge branch 'cleanup'

Over the years the SPL code bases has accumulated compatibly code
to allow it to build against a wide range of Linux kernels. In
general this is desirable because it makes the code flexible.
However, once support for these old kernels is no longer needed
and is no longer being actively tested it should be removed. This
helps keep the code simple and understandable.

The spl-0.6.x releases have supported kernels all the way back to
2.6.26. This patch stack moves that cut off up to 2.6.32 and newer
kernels. This ensures we still support all the major enterprise
distributions which are largely locked in to 2.6.32 based kernels.
And at the same time we can shed a large amount of compatibility
code which simplifies maintenance and new development.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #395