selftests, x86, protection_keys: fix wrong offset in siginfo

The siginfo contains a bunch of information about the fault.
For protection keys, it tells us which protection key's
permissions were violated.

The wrong offset in here leads to reading garbage and thus
failures in the tests.

We should probably eventually move this over to using the
kernel's headers defining the siginfo instead of a hard-coded
offset.  But, for now, just do the simplest fix.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>