x86, ds: fix buffer alignment in debug store selftest

The debug store selftest code uses a stack-allocated buffer, which is
not necessarily correctly aligned.

For tests using a buffer to hold a single entry, the buffer that is
passed to ds_request must already be suitably aligned.

Pass a suitably aligned portion of the bigger buffer.

[ Impact: fix hw-branch-tracer self-test failure ]

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: markus.t.metzger@gmail.com
LKML-Reference: <20090424094309.A30145@sedona.ch.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>