Fix some paging related bugs exposed by the spawn stress test.

- Process::exec() needs to restore the original paging scope when called
  on a non-current process.
- Add missing InterruptDisabler guards around g_processes access.
- Only flush the TLB when modifying the active page tables.

Kernel/makeuserland.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+sudo id
+
+make -C ../LibC clean && \
+make -C ../LibC && \
+make -C ../Userland clean && \
+make -C ../Userland && \
+sudo ./sync.sh
+