Kernel: Implement MemoryRegion pinning

This allows process to pin a MemoryRegion into users memory space for
syscall duration without allowing user to munmap the region.

kernel/kernel/Memory/MemoryRegion.cpp

@@ -13,6 +13,7 @@ namespace Kernel
 
 	MemoryRegion::~MemoryRegion()
 	{
+		ASSERT(m_pinned_count == 0);
 		if (m_vaddr)
 			m_page_table.unmap_range(m_vaddr, m_size);
 	}

kernel/kernel/Process.cpp

@@ -1806,7 +1806,10 @@ namespace Kernel
 			const vaddr_t region_s = region->vaddr();
 			const vaddr_t region_e = region->vaddr() + region->size();
 			if (vaddr <= region_s && region_e <= vaddr + len)
+			{
+				region->wait_not_pinned();
 				m_mapped_regions.remove(i--);
+			}
 			else if (region->overlaps(vaddr, len))
 				dwarnln("TODO: partial region munmap");
 		}