MIPS: scall64-o32: Fix indirect syscall detection

Commit 4c21b8fd8f (MIPS: seccomp: Handle indirect system calls (o32))
added indirect syscall detection for O32 processes running on MIPS64
but it did not work as expected. The reason is the the scall64-o32
implementation differs compared to scall32-o32. In the former, the v0
(syscall number) register contains the absolute syscall number
(4000 + X) whereas in the latter it contains the relative syscall
number (X). Fix the code to avoid doing an extra addition, and load
the v0 register directly to the first argument for syscall_trace_enter.
Moreover, set the .reorder assembler option in order to have better
control on this part of the assembly code.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: http://patchwork.linux-mips.org/patch/7481/
Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: James Hogan <james.hogan@imgtec.com>

arch/mips/kernel/scall64-o32.S

@@ -113,15 +113,19 @@ trace_a_syscall:
 	move	s0, t2			# Save syscall pointer
 	move	a0, sp
 	/*
-	 * syscall number is in v0 unless we called syscall(__NR_###)
+	 * absolute syscall number is in v0 unless we called syscall(__NR_###)
 	 * where the real syscall number is in a0
 	 * note: NR_syscall is the first O32 syscall but the macro is
 	 * only defined when compiling with -mabi=32 (CONFIG_32BIT)
 	 * therefore __NR_O32_Linux is used (4000)
 	 */
-	addiu	a1, v0,  __NR_O32_Linux
-	bnez	v0, 1f /* __NR_syscall at offset 0 */
-	lw	a1, PT_R4(sp)
+	.set	push
+	.set	reorder
+	subu	t1, v0,  __NR_O32_Linux
+	move	a1, v0
+	bnez	t1, 1f /* __NR_syscall at offset 0 */
+	lw	a1, PT_R4(sp) /* Arg1 for __NR_syscall case */
+	.set	pop
 
 1:	jal	syscall_trace_enter