tile/ptrace: Preserve previous registers for short regset write

commit fd7c9914 upstream.

Ensure that if userspace supplies insufficient data to
PTRACE_SETREGSET to fill all the registers, the thread's old
registers are preserved.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/tile/kernel/ptrace.c

@@ -110,7 +110,7 @@ static int tile_gpr_set(struct task_struct *target,
 			  const void *kbuf, const void __user *ubuf)
 {
 	int ret;
-	struct pt_regs regs;
+	struct pt_regs regs = *task_pt_regs(target);
 
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &regs, 0,
 				 sizeof(regs));