fib: rules: remove repeated assignment in fib_nl2rule

In fib_nl2rule(), 'err' variable has been set to -EINVAL during
declaration, and no need to set the 'err' variable to -EINVAL again.
So, remove it.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>