metag/ptrace: Reject partial NT_METAG_RPIPE writes

commit 7195ee3120d878259e8d94a5d9f808116f34d5ea upstream.

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: Dave Martin <Dave.Martin@arm.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>