unix_bind(): separate BSD and abstract cases

mainline inclusion
from mainline-v5.14-rc1
commit aee51517
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4OM1C
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=aee515170576609a0aa3413dc06a7f36f05a5fe2



--------------------------------

We do get some duplication that way, but it's minor compared to
parts that are different.  What we get is an ability to change
locking in BSD case without making failure exits very hard to
follow.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>