dump: don't play with a function exit code

We should not have a chance to exit with a wrong code on error paths.

Now dump_one_task() returs zero, if allocation of dfds failed:

ret = collect_mappings(pid, &vmas);
if (ret) {
	pr_err("Collect mappings (pid: %d) failed with %d\n", pid, ret);
	goto err;
}

if (!shared_fdtable(item)) {
	dfds = xmalloc(sizeof(*dfds));
	if (!dfds)
		goto err;

...
err:
	return -1;

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>