flock: blocked processes are not interesting for us

All out processes are stopped in a moment, when file locks are
collected, so they can't to wait any locks.

Here is a proof of this theory:
[root@avagin-fc19-cr ~]# flock xxx sleep 1000 &
[1] 23278
[root@avagin-fc19-cr ~]# flock xxx sleep 1000 &
[2] 23280
[root@avagin-fc19-cr ~]# cat /proc/locks
1: FLOCK  ADVISORY  WRITE 23278 08:03:280001 0 EOF
1: -> FLOCK  ADVISORY  WRITE 23280 08:03:280001 0 EOF
[root@avagin-fc19-cr ~]# gdb -p 23280
(gdb) ^Z
[3]+  Stopped                 gdb -p 23280
[root@avagin-fc19-cr ~]# cat /proc/locks
1: FLOCK  ADVISORY  WRITE 23278 08:03:280001 0 EOF

Currently criu can dump nothing, if we have one process which is
waiting a lock. I don't see any reason to do this.

Cc: Qiang Huang <h.huangqiang@huawei.com>
Reported-by: Mr Jenkins