Cleanly remove spl-modules-devel headers

Add the /usr/src/spl-<version>-<release>/<kernel> directory to
the spl-modules-devel package. This ensures that this directory
will be removed when the package is removed.

We do not include the higher level /usr/src/spl-<version>-<release>
directory since there may be builds for multiple kernels. Instead,
a %postun rmdir is added which attempts to remove this directory.
It will only succeed when the last spl-modules-devel-* package
for this specific release is removed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>