fix: CompactMap race condition causing runtime panic (#8029)

Fixed critical race condition in CompactMap where Set(), Delete(), and
Get() methods had issues with concurrent map access.

Root cause: segmentForKey() can create new map segments, which modifies
the cm.segments map. Calling this under a read lock caused concurrent
map write panics when multiple goroutines accessed the map simultaneously
(e.g., during VolumeNeedleStatus gRPC calls).

Changes:
- Set() method: Changed RLock/RUnlock to Lock/Unlock
- Delete() method: Changed RLock/RUnlock to Lock/Unlock, optimized to
  avoid creating empty segments when key doesn't exist
- Get() method: Removed segmentForKey() call to avoid race condition,
  now checks segment existence directly and returns early if segment
  doesn't exist (optimization: avoids unnecessary segment creation)

This fix resolves the runtime/maps.fatal panic that occurred under
concurrent load.

Tested with race detector: go test -v -race ./weed/storage/needle_map/...

weed/storage/needle_map/compact_map.go

@@ -236,8 +236,8 @@ func (cm *CompactMap) segmentForKey(key types.NeedleId) *CompactMapSegment {
 // Set inserts/updates a NeedleValue.
 // If the operation is an update, returns the overwritten value's previous offset and size.
 func (cm *CompactMap) Set(key types.NeedleId, offset types.Offset, size types.Size) (oldOffset types.Offset, oldSize types.Size) {
-	cm.RLock()
-	defer cm.RUnlock()
+	cm.Lock()
+	defer cm.Unlock()
 
 	cs := cm.segmentForKey(key)
 	return cs.set(key, offset, size)
@@ -248,7 +248,11 @@ func (cm *CompactMap) Get(key types.NeedleId) (*NeedleValue, bool) {
 	cm.RLock()
 	defer cm.RUnlock()
 
-	cs := cm.segmentForKey(key)
+	chunk := Chunk(key / SegmentChunkSize)
+	cs, ok := cm.segments[chunk]
+	if !ok {
+		return nil, false
+	}
 	if cnv, found := cs.get(key); found {
 		nv := cnv.NeedleValue(cs.chunk)
 		return &nv, true
@@ -258,10 +262,14 @@ func (cm *CompactMap) Get(key types.NeedleId) (*NeedleValue, bool) {
 
 // Delete deletes a map entry by key. Returns the entries' previous Size, if available.
 func (cm *CompactMap) Delete(key types.NeedleId) types.Size {
-	cm.RLock()
-	defer cm.RUnlock()
+	cm.Lock()
+	defer cm.Unlock()
 
-	cs := cm.segmentForKey(key)
+	chunk := Chunk(key / SegmentChunkSize)
+	cs, ok := cm.segments[chunk]
+	if !ok {
+		return types.Size(0)
+	}
 	return cs.delete(key)
 }