kernel: Add reference counting for mounts

This will make sure we cannot unmount a file system while there is another one mounted inside it.

kernel/src/fs/Mount.cpp

@@ -13,7 +13,14 @@ Result<SharedPtr<VFS::Inode>> MountInode::create(SharedPtr<VFS::Inode> source, S
     auto parent = TRY(source->find(".."));
     TRY(fs->set_mount_dir(parent));
 
+    source->add_handle();
+
     g_mounts.append(inode.ptr());
 
     return (SharedPtr<VFS::Inode>)inode;
 }
+
+MountInode::~MountInode()
+{
+    m_source->remove_handle();
+}

kernel/src/fs/Mount.h

@@ -137,7 +137,7 @@ class MountInode : public VFS::Inode, public LinkedListNode<MountInode>
         return m_mount_root_inode->replace_entry(inode, name);
     }
 
-    virtual ~MountInode() = default;
+    virtual ~MountInode();
 
   private:
     SharedPtr<VFS::Inode> m_source;

kernel/src/fs/VFS.cpp

@@ -183,6 +183,7 @@ namespace VFS
 
         auto mount = (MountInode*)inode.ptr();
         TRY(parent_inode->replace_entry(mount->source(), child.chars()));
+        g_mounts.remove(mount);
 
         return {};
     }