kernel: remove todo!() on NonfatalInternalError in page fault handler

The page fault handler for user-mode faults had a separate
arms for Segv, RecursionLimitExceeded, and NonfatalInternalError from
try_correcting_page_tables. The first two fell through to return
Segv to the process; NonfatalInternalError called todo!() which
causes a kernel panic. This is a kernel-level crash triggered by
a userspace page table correction attempt that reports an internal
(non-fatal) error.

Fix: collapse NonfatalInternalError into the same fall-through arm
as Segv and RecursionLimitExceeded. The error name says 'nonfatal'
— it should not crash the kernel. The userspace process receives
a segmentation fault signal instead.

src/memory/mod.rs

@@ -1028,8 +1028,7 @@ pub fn page_fault_handler(
         match context::memory::try_correcting_page_tables(faulting_page, mode, &mut token) {
             Ok(()) => return Ok(()),
             Err(PfError::Oom) => todo!("oom"),
-            Err(PfError::Segv | PfError::RecursionLimitExceeded) => (),
-            Err(PfError::NonfatalInternalError) => todo!(),
+            Err(PfError::Segv | PfError::RecursionLimitExceeded | PfError::NonfatalInternalError) => (),
         }
     }