Kernel: Update atime/ctime/mtime timestamps atomically

Instead of having three separate APIs (one for each timestamp),
there's now only Inode::update_timestamps() and it takes 3x optional
timestamps. The non-empty timestamps are updated while holding the inode
mutex, and the outside world no longer has to look at intermediate
timestamp states.

Kernel/FileSystem/ISO9660FileSystem.cpp

@@ -528,17 +528,7 @@ ErrorOr<void> ISO9660Inode::truncate(u64)
     return EROFS;
 }
 
-ErrorOr<void> ISO9660Inode::set_atime(time_t)
-{
-    return EROFS;
-}
-
-ErrorOr<void> ISO9660Inode::set_ctime(time_t)
-{
-    return EROFS;
-}
-
-ErrorOr<void> ISO9660Inode::set_mtime(time_t)
+ErrorOr<void> ISO9660Inode::update_timestamps(Optional<time_t>, Optional<time_t>, Optional<time_t>)
 {
     return EROFS;
 }