x86/pkeys/selftests: Fix pointer math

x86/pkeys/selftests: Fix pointer math

We dump out the entire area of the siginfo where the si_pkey_ptr is
supposed to be.  But, we do some math on the poitner, which is a u32.
We intended to do byte math, not u32 math on the pointer.

Cast it over to a u8* so it works.

Also, move this block of code to below th si_code check.  It doesn't
hurt anything, but the si_pkey field is gibberish for other signal
types.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellermen <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20180509171352.9BE09819@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>