Kernel/FileSystem: Enforce locking of m_inode_lock when truncating Inode

Such operation is almost equivalent to writing on an Inode, so lock the
Inode m_inode_lock exclusively.
All FileSystem Inode implementations then override a new method called
truncate_locked which should implement the actual truncating.

Kernel/FileSystem/ISO9660FS/Inode.cpp

@@ -140,7 +140,7 @@ ErrorOr<void> ISO9660Inode::chown(UserID, GroupID)
     return EROFS;
 }
 
-ErrorOr<void> ISO9660Inode::truncate(u64)
+ErrorOr<void> ISO9660Inode::truncate_locked(u64)
 {
     return EROFS;
 }