fix: enforce XATTR_REPLACE semantics in setxattr (#10059)

* 修复weedfs_xattr.go 中 XATTR_REPLACE 语义缺失

* mount: fix XATTR_CREATE/XATTR_REPLACE flag semantics in setxattr

XATTR_CREATE fell through into the XATTR_REPLACE branch: creating a new
attribute hit the empty-oldData guard and returned ENODATA instead of
creating it, while creating over an existing attribute silently succeeded
without the EEXIST that setxattr(2) requires. Drop the fallthrough chain
so CREATE returns EEXIST when the attribute already exists, REPLACE
returns ENODATA when it is missing, and otherwise the value is written.
Test existence via the map lookup so an attribute with an empty value is
still treated as present.

---------

Co-authored-by: 王郁文 <wangyuwen@cmict.chinamobile.com>
Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/mount/weedfs_xattr.go

@@ -121,22 +121,23 @@ func (wfs *WFS) SetXAttr(cancel <-chan struct{}, input *fuse.SetXAttrIn, attr st
 	if entry.Extended == nil {
 		entry.Extended = make(map[string][]byte)
 	}
-	oldData, _ := entry.Extended[XATTR_PREFIX+attr]
+	_, exists := entry.Extended[XATTR_PREFIX+attr]
 	switch input.Flags {
 	case sys.XATTR_CREATE:
-		if len(oldData) > 0 {
-			break
+		if exists {
+			return fuse.Status(syscall.EEXIST)
 		}
-		fallthrough
 	case sys.XATTR_REPLACE:
-		fallthrough
-	default:
-		// data aliases the FUSE request's pooled input buffer, which is
-		// recycled once this handler returns. Copy before storing so a
-		// later request reusing the buffer cannot corrupt the value.
-		entry.Extended[XATTR_PREFIX+attr] = append([]byte(nil), data...)
+		if !exists {
+			return fuse.ENODATA
+		}
 	}
 
+	// data aliases the FUSE request's pooled input buffer, which is
+	// recycled once this handler returns. Copy before storing so a
+	// later request reusing the buffer cannot corrupt the value.
+	entry.Extended[XATTR_PREFIX+attr] = append([]byte(nil), data...)
+
 	if fh != nil {
 		fh.dirtyMetadata = true
 		return fuse.OK