unix: Fix double restoring of peerless unix sockets, which are peers for others

Since commit ea747b07 receive queue for
such DGRAM sockets restores twice: in open_unix_sk() and post_open_unix_sk().
It should be made only once. So, keep that commit logic only for sockets
without alive sender.

Signed-off-by: Kirill Tkhai <ktkhai@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>