Kernel: Add const variant of Process::perf_events()

This lets us get rid of some const_casts.

Kernel/Process.h

@@ -503,6 +503,7 @@ public:
     const NonnullRefPtrVector<Thread>& threads_for_coredump(Badge<Coredump>) const { return m_threads_for_coredump; }
 
     PerformanceEventBuffer* perf_events() { return m_perf_event_buffer; }
+    PerformanceEventBuffer const* perf_events() const { return m_perf_event_buffer; }
 
     Memory::AddressSpace& address_space() { return *m_space; }
     Memory::AddressSpace const& address_space() const { return *m_space; }

Kernel/ProcessSpecificExposed.cpp

@@ -162,11 +162,11 @@ KResult Process::procfs_get_unveil_stats(KBufferBuilder& builder) const
 KResult Process::procfs_get_perf_events(KBufferBuilder& builder) const
 {
     InterruptDisabler disabler;
-    if (!const_cast<Process&>(*this).perf_events()) {
+    if (!perf_events()) {
         dbgln("ProcFS: No perf events for {}", pid());
         return KResult(ENOBUFS);
     }
-    return const_cast<Process&>(*this).perf_events()->to_json(builder) ? KSuccess : KResult(EINVAL);
+    return perf_events()->to_json(builder) ? KSuccess : KResult(EINVAL);
 }
 
 KResult Process::procfs_get_fds_stats(KBufferBuilder& builder) const