powerpc: Rework syscall_get_arguments() so there is only one loop

Currently syscall_get_arguments() has two loops, one for compat and one
for regular tasks. In prepartion for the next patch, which changes which
registers we use, switch it to only have one loop, so we only have one
place to update.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>

arch/powerpc/include/asm/syscall.h

@@ -64,19 +64,16 @@ static inline void syscall_get_arguments(struct task_struct *task,
 					 unsigned int i, unsigned int n,
 					 unsigned long *args)
 {
+	unsigned long mask = -1UL;
+
 	BUG_ON(i + n > 6);
-#ifdef CONFIG_PPC64
-	if (test_tsk_thread_flag(task, TIF_32BIT)) {
-		/*
-		 * Zero-extend 32-bit argument values.  The high bits are
-		 * garbage ignored by the actual syscall dispatch.
-		 */
-		while (n-- > 0)
-			args[n] = (u32) regs->gpr[3 + i + n];
-		return;
-	}
+
+#ifdef CONFIG_COMPAT
+	if (test_tsk_thread_flag(task, TIF_32BIT))
+		mask = 0xffffffff;
 #endif
-	memcpy(args, &regs->gpr[3 + i], n * sizeof(args[0]));
+	while (n--)
+		args[n] = regs->gpr[3 + i + n] & mask;
 }
 
 static inline void syscall_set_arguments(struct task_struct *task,