fs/inode: replace some global variables to macro

replace to macro will help to extend the scheduling implementation

Signed-off-by: chao an <anchao@lixiang.com>

fs/inode/fs_foreachinode.c

@@ -187,7 +187,7 @@ int foreach_inode(foreach_inode_t handler, FAR void *arg)
   ret = inode_lock();
   if (ret >= 0)
     {
-      ret = foreach_inodelevel(g_root_inode->i_child, info);
+      ret = foreach_inodelevel(inode_root()->i_child, info);
       inode_unlock();
     }
 
@@ -211,7 +211,7 @@ int foreach_inode(foreach_inode_t handler, FAR void *arg)
   ret = inode_lock();
   if (ret >= 0)
     {
-      ret = foreach_inodelevel(g_root_inode->i_child, &info);
+      ret = foreach_inodelevel(inode_root()->i_child, &info);
       inode_unlock();
     }
 

fs/inode/fs_inodereserve.c

@@ -141,7 +141,7 @@ static void inode_insert(FAR struct inode *node,
 
 void inode_root_reserve(void)
 {
-  g_root_inode = inode_alloc("", 0777);
+  inode_root() = inode_alloc("", 0777);
 }
 
 /****************************************************************************

fs/inode/fs_inodesearch.c

@@ -214,7 +214,7 @@ static int _inode_linktarget(FAR struct inode *node,
 static int _inode_search(FAR struct inode_search_s *desc)
 {
   FAR const char   *name;
-  FAR struct inode *node    = g_root_inode;
+  FAR struct inode *node    = inode_root();
   FAR struct inode *left    = NULL;
   FAR struct inode *above   = NULL;
   FAR const char   *relpath = NULL;

fs/inode/inode.h

@@ -65,6 +65,8 @@
     } \
   while (0)
 
+#define inode_root() g_root_inode
+
 /****************************************************************************
  * Public Types
  ****************************************************************************/