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/SysFS/Inode.cpp

@@ -105,8 +105,9 @@ ErrorOr<void> SysFSInode::chown(UserID, GroupID)
     return EPERM;
 }
 
-ErrorOr<void> SysFSInode::truncate(u64 size)
+ErrorOr<void> SysFSInode::truncate_locked(u64 size)
 {
+    VERIFY(m_inode_lock.is_locked());
     return m_associated_component->truncate(size);
 }