eth/protocols/snap: fix batch writer when resuming an aborted sync (#22)

### Description

upstream PR:
[go-ethereum#27842](ethereum/go-ethereum#27842)

Fixes ethereum/go-ethereum#27813.

The closure caused all subtasks to reference the same one batch when
writing, so instead of each subtask of a contract growing and flushing
it's own batch, they all grew the last one's, but flushed their own.

Interestingly however, this should only be an issue if there's also a
lack of peers, otherwise whenever a delivery into the last batch is
made, it should have flushed out everyone else's data too.

eth/protocols/snap/sync.go

@@ -736,6 +736,8 @@ func (s *Syncer) loadSyncStatus() {
 			}
 			s.tasks = progress.Tasks
 			for _, task := range s.tasks {
+				task := task // closure for task.genBatch in the stacktrie writer callback
+
 				task.genBatch = ethdb.HookedBatch{
 					Batch: s.db.NewBatch(),
 					OnPut: func(key []byte, value []byte) {
@@ -746,6 +748,8 @@ func (s *Syncer) loadSyncStatus() {
 
 				for _, subtasks := range task.SubTasks {
 					for _, subtask := range subtasks {
+						subtask := subtask // closure for subtask.genBatch in the stacktrie writer callback
+
 						subtask.genBatch = ethdb.HookedBatch{
 							Batch: s.db.NewBatch(),
 							OnPut: func(key []byte, value []byte) {