ipv4: Fix error return code in fib_table_insert()

In fib_table_insert(), if the alias was already inserted, but node not
exist, the error code should be set before return from error handling path.

Fixes: a6c76c17 ("ipv4: Notify route after insertion to the routing table")
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Link: https://lore.kernel.org/r/20221120072838.2167047-1-william.xuanziyang@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>