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: c0ff3c53d4f99 ("MIPS: Enable HAVE_ARCH_TRACEHOOK.")
Signed-off-by: Dmitry V. Levin <[e-mail]>
Signed-off-by: Paul Burton <[e-mail]>
Patchwork: https://patchwork.linux-mips.org/patch/21185/
Cc: Elvira Khabirova <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: James Hogan <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail] # v3.13+

arch/mips/include/asm/syscall.h

···
 #ifdef CONFIG_64BIT
 	case 4: case 5: case 6: case 7:
 #ifdef CONFIG_MIPS32_O32
-		if (test_thread_flag(TIF_32BIT_REGS))
+		if (test_tsk_thread_flag(task, TIF_32BIT_REGS))
 			return get_user(*arg, (int *)usp + n);
 		else
 #endif