mips: fix mips_get_syscall_arg o32 check

When checking for TIF_32BIT_REGS flag, mips_get_syscall_arg() should
use the task specified as its argument instead of the current task.

This potentially affects all syscall_get_arguments() users
who specify tasks different from the current.

Fixes: c0ff3c53 ("MIPS: Enable HAVE_ARCH_TRACEHOOK.")
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21185/
Cc: Elvira Khabirova <lineprinter@altlinux.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org # v3.13+