flock: Merge all file lock entries into single image file

They are now in per-pid images, but every entry contains a
pid to which it "belongs". This belonging is fake -- it's
just a pid of a task who placed the lock, while locks really
belong to files. We even have a bug when task that locked
a file exited and "delegated" the lock to its child.

This images merge reduces the amount of image files criu
generates and may simplify the fix of mentioned above issue.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>