ipv6: Always allocate pcpu memory in a fib6_nh

A recent commit had an unintended side effect with reject routes:
rt6i_pcpu is expected to always be initialized for all fib6_info except
the null entry. The commit mentioned below skips it for reject routes
and ends up leaking references to the loopback device. For example,

    ip netns add foo
    ip -netns foo li set lo up
    ip -netns foo -6 ro add blackhole 2001:db8:1::1
    ip netns exec foo ping6 2001:db8:1::1
    ip netns del foo

ends up spewing:
    unregister_netdevice: waiting for lo to become free. Usage count = 3

The fib_nh_common_init is not needed for reject routes (no ipv4 caching
or encaps), so move the alloc_percpu_gfp after it and adjust the goto label.

Fixes: f40b6ae2

 ("ipv6: Move pcpu cached routes to fib6_nh")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>