af_unix: Pass struct sock to unix_autobind().

We do not use struct socket in unix_autobind() and pass struct sock to
unix_bind_bsd() and unix_bind_abstract().  Let's pass it to unix_autobind()
as well.

Also, this patch fixes these errors by checkpatch.pl.

  ERROR: do not use assignment in if condition
  #1795: FILE: net/unix/af_unix.c:1795:
  +	if (test_bit(SOCK_PASSCRED, &sock->flags) && !u->addr

  CHECK: Logical continuations should be on the previous line
  #1796: FILE: net/unix/af_unix.c:1796:
  +	if (test_bit(SOCK_PASSCRED, &sock->flags) && !u->addr
  +	    && (err = unix_autobind(sock)) != 0)

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>