uprobes: Change uprobe_copy_process() to dup return_instances

uprobe_copy_process() assumes that the new child doesn't need
->utask, it should be allocated by demand.

But this is not true if the forking task has the pending ret-
probes, the child should report them as well and thus it needs
the copy of parent's ->return_instances chain. Otherwise the
child crashes when it returns from the probed function.

Alternatively we could cleanup the child's stack, but this needs
per-arch changes and this is not what we want. At least systemtap
expects a .return in the child too.

Note: this change alone doesn't fix the problem, see the next
change.

Reported-by: Martin Cermak <[email protected]>
Reported-by: David Smith <[email protected]>
Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Srikar Dronamraju <[email protected]>

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