powerpc/audit: Fix syscall_get_arch()

Commit 770cec16 ("powerpc/audit: Simplify syscall_get_arch()")
and commit 898a1ef0 ("powerpc/audit: Avoid unneccessary #ifdef
in syscall_get_arguments()")
replaced test_tsk_thread_flag(task, TIF_32BIT)) by is_32bit_task().

But is_32bit_task() applies on current task while be want the test
done on task 'task'

So add a new macro is_tsk_32bit_task() to check any task.

Fixes: 770cec16 ("powerpc/audit: Simplify syscall_get_arch()")
Fixes: 898a1ef0

 ("powerpc/audit: Avoid unneccessary #ifdef in syscall_get_arguments()")
Cc: stable@vger.kernel.org
Reported-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c55cddb8f65713bf5859ed675d75a50cb37d5995.1642159570.git.christophe.leroy@csgroup.eu