mctp: Fix an error handling path in mctp_init()

If mctp_neigh_init() return error, the routes resources should
be released in the error handling path. Otherwise some resources
leak.

Fixes: 4d8b9319

 ("mctp: Add neighbour implementation")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Matt Johnston <matt@codeconstruct.com.au>
Link: https://lore.kernel.org/r/20221108095517.620115-1-weiyongjun@huaweicloud.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>