kernel: Make pivot_root() reset the parent entry of the new root directory

Otherwise it would just be pointing to the old parent fs, and we don't want that.

kernel/src/fs/VFS.cpp

@@ -217,6 +217,7 @@ namespace VFS
         g_root_inode = new_root_inode;
         TRY(new_root_parent_inode->replace_entry(((MountInode*)g_root_inode.ptr())->source(), new_root_path.chars()));
         ((MountInode*)g_root_inode.ptr())->set_source({});
+        g_root_inode->fs()->reset_mount_dir();
 
         return {};
     }

kernel/src/fs/VFS.h

@@ -36,6 +36,8 @@ namespace VFS
 
         virtual Result<void> set_mount_dir(SharedPtr<Inode> parent) = 0;
 
+        virtual Result<void> reset_mount_dir() = 0;
+
         virtual bool is_readonly() const
         {
             return false;

kernel/src/fs/ext2/FileSystem.cpp

@@ -125,4 +125,9 @@ namespace Ext2
     {
         return m_root_inode->replace_entry(inode, "..");
     }
+
+    Result<void> FileSystem::reset_mount_dir()
+    {
+        return m_root_inode->replace_entry(m_root_inode, "..");
+    }
 }

kernel/src/fs/ext2/FileSystem.h

@@ -149,6 +149,8 @@ namespace Ext2
 
         Result<void> set_mount_dir(SharedPtr<VFS::Inode>) override;
 
+        Result<void> reset_mount_dir() override;
+
         bool is_readonly() const override
         {
             return true;

kernel/src/fs/tmpfs/FileSystem.cpp

@@ -72,6 +72,11 @@ namespace TmpFS
         return m_root_inode->replace_entry(parent, "..");
     }
 
+    Result<void> FileSystem::reset_mount_dir()
+    {
+        return m_root_inode->replace_entry(m_root_inode, "..");
+    }
+
     void FileSystem::set_root(SharedPtr<VFS::Inode> root)
     {
         m_root_inode = root;

kernel/src/fs/tmpfs/FileSystem.h

@@ -20,6 +20,8 @@ namespace TmpFS
 
         Result<void> set_mount_dir(SharedPtr<VFS::Inode> parent) override;
 
+        Result<void> reset_mount_dir() override;
+
         static Result<SharedPtr<VFS::FileSystem>> create();
 
         dev_t host_device_id() const override