storage: fix possible raft log panic after fsync error

Detected with #36989 applied by running `./bin/roachtest run --local '^system-crash/sync-errors=true$'`.
With some slight modification to that test's constants it could repro
errors like this within a minute:

```
panic: tocommit(375) is out of range [lastIndex(374)]. Was the raft log corrupted, truncated, or lost?
```

Debugging showed `DBSyncWAL` can be called even after a sync failure.
I guess if it returns success any time after it fails it will ack
writes that aren't recoverable in WAL. They aren't recoverable because
RocksDB stops recovery upon hitting the offset corresponding to the
lost write (typically there should be a corruption there). Meanwhile,
there are still successfully synced writes at later offsets in the
file.

The fix is simple. If `DBSyncWAL` returns an error once, keep track of
that error and return it for all future writes.

Release note: None

pkg/storage/engine/rocksdb.go

@@ -688,6 +688,7 @@ func (r *RocksDB) syncLoop() {
 	s.Lock()
 
 	var lastSync time.Time
+	var err error
 
 	for {
 		for len(s.pending) == 0 && !s.closed {
@@ -713,8 +714,7 @@ func (r *RocksDB) syncLoop() {
 
 		s.Unlock()
 
-		var err error
-		if r.cfg.Dir != "" {
+		if r.cfg.Dir != "" && err == nil {
 			err = statusToError(C.DBSyncWAL(r.rdb))
 			lastSync = timeutil.Now()
 		}