netdevsim: fib: Remove redundant code

Fix the following coccicheck warnings:

./drivers/net/netdevsim/fib.c:874:5-8: Unneeded variable: "err". Return
"0" on line 889.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>