net: unix: remove redundant assignment to variable 'err'

After commit 37ab4fa7 ("net: unix: allow bind to fail on mutex lock"),
the assignment to err is redundant. So remove it.

Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>