rafs: fix bug in InodeWrapper::is_sock()

We incorrectly use is_dir() to check if a file is a socket. This patch
fixes it.

Signed-off-by: Yifan Zhao <zhaoyifan@sjtu.edu.cn>

rafs/src/metadata/inode.rs

@@ -183,7 +183,7 @@ impl InodeWrapper {
         match self {
             InodeWrapper::V5(i) => i.is_sock(),
             InodeWrapper::V6(i) => i.is_sock(),
-            InodeWrapper::Ref(i) => i.as_inode().is_dir(),
+            InodeWrapper::Ref(i) => i.as_inode().is_sock(),
         }
     }