Revert "Kernel: Protect InodeWatcher internals with spinlock instead of mutex"

This reverts commit 0bebf013e3.

This caused a deadlock when handling a crashed process, so let's revert
it until we can figure out what went wrong.

Kernel/FileSystem/InodeWatcher.h

@@ -64,7 +64,7 @@ public:
 private:
     explicit InodeWatcher() { }
 
-    mutable Spinlock m_lock;
+    mutable Mutex m_lock;
 
     struct Event {
         int wd { 0 };