Ext2FS: Trying to create a too-long directory entry should ENAMETOOLONG

Also added some assertions to DirectoryEntry in case someone tries to
instantiate them with names that would overflow the name buffer.

DirectoryEntry is a crappy data structure, and the name buffer is also
crappy. Added a FIXME about replacing it with something nicer.

Before this patch, the DirectoryEntry::name buffer would overflow if
you did "touch extremely-long-file-name". Duh.

Fixes #538.

Kernel/FileSystem/FileSystem.h

@@ -47,6 +47,7 @@ public:
 
     virtual KResult prepare_to_unmount() const { return KSuccess; }
 
+    // FIXME: This data structure is very clunky and unpleasant. Replace it with something nicer.
     struct DirectoryEntry {
         DirectoryEntry(const char* name, InodeIdentifier, u8 file_type);
         DirectoryEntry(const char* name, int name_length, InodeIdentifier, u8 file_type);