mips: take the "zero newsp means inherit the parent's one" to copy_thread()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index a11c6f9..a33d2ef 100644
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -156,7 +156,8 @@
 	*childregs = *regs;
 	childregs->regs[7] = 0;	/* Clear error flag */
 	childregs->regs[2] = 0;	/* Child gets zero as return value */
-	childregs->regs[29] = usp;
+	if (usp)
+		childregs->regs[29] = usp;
 	ti->addr_limit = USER_DS;
 
 	p->thread.reg29 = (unsigned long) childregs;