pstree: try to find a free pid between busy pids (v2)

Currently our pid allocator returns max_pid++ and it can return a pid
which is bigger than kernel.max_pid.

(00.821430)   5506: Error (cr-restore.c:1540): Pid 300 do not match expected 32768

v2: handle error code from insert_item()
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>