net: ipip: fix wrong address family in init error path

[ Upstream commit 57ebc8f08504f176eb0f25b3e0fde517dec61a4f ]

In case of error with MPLS support the code is misusing AF_INET
instead of AF_MPLS.

Fixes: 1b69e7e6 ("ipip: support MPLS over IPv4")
Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>