icmp: allow icmpv6_ndo_send to work with CONFIG_IPV6=n

commit a8e41f60

 upstream.

The icmpv6_send function has long had a static inline implementation
with an empty body for CONFIG_IPV6=n, so that code calling it doesn't
need to be ifdef'd. The new icmpv6_ndo_send function, which is intended
for drivers as a drop-in replacement with an identical function
signature, should follow the same pattern. Without this patch, drivers
that used to work with CONFIG_IPV6=n now result in a linker error.

Cc: Chen Zhou <chenzhou10@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 0b41713b

 ("icmp: introduce helper for nat'd source address in network device context")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>