metag/ptrace: Reject partial NT_METAG_RPIPE writes

It's not clear what behaviour is sensible when doing partial write of
NT_METAG_RPIPE, so just don't bother.

This patch assumes that userspace will never rely on a partial SETREGSET
in this case, since it's not clear what should happen anyway.
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Acked-by: NJames Hogan <james.hogan@imgtec.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/metag/kernel/ptrace.c

@@ -253,6 +253,8 @@ int metag_rp_state_copyin(struct pt_regs *regs,
 	unsigned long long *ptr;
 	int ret, i;
 
+	if (count < 4*13)
+		return -EINVAL;
 	/* Read the entire pipeline before making any changes */
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
 				 &rp, 0, 4*13);