powerpc/ptrace: Fix out of bounds array access warning

gcc-6 correctly warns about a out of bounds access

arch/powerpc/kernel/ptrace.c:407:24: warning: index 32 denotes an offset greater than size of 'u64[32][1] {aka long long unsigned int[32][1]}' [-Warray-bounds]
        offsetof(struct thread_fp_state, fpr[32][0]));
                        ^

check the end of array instead of beginning of next element to fix this

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>