MIPS: R2-on-R6 MULTU/MADDU/MSUBU emulation bugfix

MIPS instructions MULTU, MADDU and MSUBU emulation requires registers HI/LO
to be converted to signed 32bits before 64bit sign extension on MIPS64.

Bug was found on running MIPS32 R2 test application on MIPS64 R6 kernel.

Fixes: b0a668fb2038 ("MIPS: kernel: mips-r2-to-r6-emul: Add R2 emulator for MIPS R6")
Signed-off-by: Leonid Yegoshin <[e-mail]>
Reported-by: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/14043/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/kernel/mips-r2-to-r6-emul.c

···
 	rs = regs->regs[MIPSInst_RS(ir)];
 	res = (u64)rt * (u64)rs;
 	rt = res;
-	regs->lo = (s64)rt;
-	regs->hi = (s64)(res >> 32);
+	regs->lo = (s64)(s32)rt;
+	regs->hi = (s64)(s32)(res >> 32);
 
 	MIPS_R2_STATS(muls);
 
···
 	res += ((((s64)rt) << 32) | (u32)rs);
 
 	rt = res;
-	regs->lo = (s64)rt;
+	regs->lo = (s64)(s32)rt;
 	rs = res >> 32;
-	regs->hi = (s64)rs;
+	regs->hi = (s64)(s32)rs;
 
 	MIPS_R2_STATS(dsps);
 
···
 	res = ((((s64)rt) << 32) | (u32)rs) - res;
 
 	rt = res;
-	regs->lo = (s64)rt;
+	regs->lo = (s64)(s32)rt;
 	rs = res >> 32;
-	regs->hi = (s64)rs;
+	regs->hi = (s64)(s32)rs;
 
 	MIPS_R2_STATS(dsps);