ipv6: return errno pointers consistently for fib6_add_1()

fib6_add_1() should consistently return errno pointers,
rather than a mixture of NULL and errno pointers.

Signed-off-by: Lin Ming <mlin@ss.pku.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/ipv6/ip6_fib.c
Simple merge