Kernel: Fix block recursion

Since the process lock is using the Lock class, re-locking the process
lock may cause another call to Thread::block. This caused some problems
with multiple blockers attempting to be used at the same time. To solve
this problem, remember if the process lock was held, and if it was then
relock after we're done with the blockers, just before returning.

Kernel/Thread.h

@@ -786,7 +786,7 @@ public:
         ScopedSpinLock block_lock(m_block_lock);
         // We need to hold m_block_lock so that nobody can unblock a blocker as soon
         // as it is constructed and registered elsewhere
-        m_in_block++;
+        m_in_block = true;
         T t(forward<Args>(args)...);
 
         Atomic<bool> timeout_unblocked(false);
@@ -809,7 +809,7 @@ public:
                 // Don't block if the wake condition is already met
                 t.not_blocking(false);
                 m_blocker = nullptr;
-                m_in_block--;
+                m_in_block = false;
                 return BlockResult::NotBlocked;
             }
 
@@ -831,7 +831,7 @@ public:
                     // Timeout is already in the past
                     t.not_blocking(true);
                     m_blocker = nullptr;
-                    m_in_block--;
+                    m_in_block = false;
                     return BlockResult::InterruptedByTimeout;
                 }
             }
@@ -844,12 +844,13 @@ public:
         block_lock.unlock();
 
         bool did_timeout = false;
-        bool last_recursive_unblock;
+        bool did_unlock = false;
         for (;;) {
             scheduler_lock.unlock();
 
             // Yield to the scheduler, and wait for us to resume unblocked.
-            m_need_relock_process |= unlock_process_if_locked();
+            did_unlock |= unlock_process_if_locked();
+
             ASSERT(!g_scheduler_lock.own_lock());
             ASSERT(Processor::current().in_critical());
             yield_while_not_holding_big_lock();
@@ -875,7 +876,7 @@ public:
                 ASSERT(m_blocker == &t);
                 m_blocker = nullptr;
             }
-            last_recursive_unblock = (--m_in_block == 0);
+            m_in_block = false;
             break;
         }
 
@@ -895,8 +896,7 @@ public:
             // (e.g. if it's on another processor)
             TimerQueue::the().cancel_timer(timer.release_nonnull());
         }
-        if (m_need_relock_process && last_recursive_unblock) {
-            m_need_relock_process = false;
+        if (did_unlock) {
             // NOTE: this may trigger another call to Thread::block(), so
             // we need to do this after we're all done and restored m_in_block!
             lock_process();
@@ -1226,12 +1226,11 @@ private:
     u32 m_priority_boost { 0 };
 
     State m_stop_state { Invalid };
-    u32 m_in_block { false };
 
     bool m_dump_backtrace_on_finalization { false };
     bool m_should_die { false };
     bool m_initialized { false };
-    bool m_need_relock_process { false };
+    bool m_in_block { false };
     Atomic<bool> m_have_any_unmasked_pending_signals { false };
 
     void yield_without_holding_big_lock();