Previously, `ln` was only set once, so search has really worked for
small number of objects.
Fix panic:
```
panic: runtime error: slice bounds out of range [:43690] with capacity 21238
goroutine 6859775 [running]:
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object.(*searchStreamMsgSizeCtrl).Send(0xc001eec8d0, 0xc005734000)
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/transport_splitter.go:173 +0x1f0
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search/v2.(*streamWriter).WriteIDs(0xc000520320, {0xc00eb1a000, 0x4fd9c, 0x7fd6475a9a68?})
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search/v2/streamer.go:28 +0x155
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search.(*uniqueIDWriter).WriteIDs(0xc001386420, {0xc00eb1a000?, 0xc0013ea9c0?, 0x113eef3?})
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search/util.go:62 +0x202
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search.(*execCtx).writeIDList(0xc00011aa38?, {0xc00eb1a000?, 0xc001eec9f0?, 0xc0008f4380?})
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search/exec.go:68 +0x91
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search.(*execCtx).executeLocal(0xc0008f4380, {0x176c538, 0xc001eec9f0})
git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/object/search/local.go:18 +0x16b
```
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>