ipv6: fix return value check in fib6_add()

In case of error, the function fib6_add_1() returns ERR_PTR()
or NULL pointer. The ERR_PTR() case check is missing in fib6_add().

dpatch engine is used to generated this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>