Ext2FS: Make Ext2FSInode::is_directory() fast

This patch overloads Inode::is_directory() with a faster version that
doesn't require instantiating the whole InodeMetadata.

If you have an Ext2FSInode&, calling is_directory() should be instant
since we can just look directly at the raw inode bits.

Kernel/FileSystem/Ext2FileSystem.h

@@ -19,6 +19,7 @@ public:
 
     size_t size() const { return m_raw_inode.i_size; }
     bool is_symlink() const { return ::is_symlink(m_raw_inode.i_mode); }
+    bool is_directory() const { return ::is_directory(m_raw_inode.i_mode); }
 
     // ^Inode (RefCounted magic)
     virtual void one_ref_left() override;