Kernel: Quickly reject userspace addresses in kernel_region_from_vaddr()

This avoids taking and releasing the MM lock just to reject an address
that we can tell from just looking at it that it won't ever be in the
kernel regions tree.

Kernel/Memory/MemoryManager.cpp

@@ -645,6 +645,9 @@ UNMAP_AFTER_INIT void MemoryManager::initialize(u32 cpu)
 
 Region* MemoryManager::kernel_region_from_vaddr(VirtualAddress vaddr)
 {
+    if (is_user_address(vaddr))
+        return nullptr;
+
     SpinlockLocker lock(s_mm_lock);
     auto* region = MM.m_kernel_regions.find_largest_not_above(vaddr.get());
     if (!region || !region->contains(vaddr))