Kernel: Stop using HashMap in Mutex

This commit removes the usage of HashMap in Mutex, thereby making Mutex
be allocation-free.

In order to achieve this several simplifications were made to Mutex,
removing unused code-paths and extra VERIFYs:
 * We no longer support 'upgrading' a shared lock holder to an
   exclusive holder when it is the only shared holder and it did not
   unlock the lock before relocking it as exclusive. NOTE: Unlike the
   rest of these changes, this scenario is not VERIFY-able in an
   allocation-free way, as a result the new LOCK_SHARED_UPGRADE_DEBUG
   debug flag was added, this flag lets Mutex allocate in order to
   detect such cases when debugging a deadlock.
 * We no longer support checking if a Mutex is locked by the current
   thread when the Mutex was not locked exclusively, the shared version
   of this check was not used anywhere.
 * We no longer support force unlocking/relocking a Mutex if the Mutex
   was not locked exclusively, the shared version of these functions
   was not used anywhere.

Kernel/Process.cpp

@@ -105,7 +105,7 @@ void Process::kill_threads_except_self()
     });
 
     u32 dropped_lock_count = 0;
-    if (big_lock().force_unlock_if_locked(dropped_lock_count) != LockMode::Unlocked)
+    if (big_lock().force_unlock_exclusive_if_locked(dropped_lock_count) != LockMode::Unlocked)
         dbgln("Process {} big lock had {} locks", *this, dropped_lock_count);
 }