net/ipv6: Enforce limits for accept_unsolicited_na sysctl

Fix mistake in the original patch where limits were specified but the
handler didn't take care of the limits.

Signed-off-by: Arun Ajith S <aajith@arista.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>