ip_tunnel: return more precise errno value when adding tunnel fails

Currently this always returns ENOBUFS, because the return value of
__ip_tunnel_create is discarded.

A more common failure is a duplicate name (EEXIST).  Propagate the real
error code so userspace can display a more meaningful error message.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>