MIPS: Correct forced syscall errors

When the system call return value is forced to be an error (for example
due to SECCOMP_RET_ERRNO), syscall_set_return_value() puts the error
code in the return register $v0 and -1 in the error register $a3.

However normally executed system calls put 1 in the error register
rather than -1, so fix syscall_set_return_value() to be consistent with
that.

I don't anticipate that anything would have been broken by this, since
the most natural way to check the error register on MIPS would be a
conditional branch if error register is [not] equal to zero (bnez or
beqz).

Fixes: 1d7bf99 ("MIPS: ftrace: Add support for syscall tracepoints.")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16652/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/syscall.h

@@ -85,7 +85,7 @@ static inline void syscall_set_return_value(struct task_struct *task,
 {
 	if (error) {
 		regs->regs[2] = -error;
-		regs->regs[7] = -1;
+		regs->regs[7] = 1;
 	} else {
 		regs->regs[2] = val;
 		regs->regs[7] = 0;