Explain why itrunc() is OK w.r.t. crashes.

fs.c

@@ -359,6 +359,8 @@ bmap(struct inode *ip, uint bn, int alloc)
 }
 
 // Truncate inode (discard contents).
+// Only called after the last dirent referring
+// to this inode has been erased on disk.
 static void
 itrunc(struct inode *ip)
 {