[kernel] Use constants for known pml4e indices

There were a few lingering bugs due to places where 510/511 were
hard-coded as the kernel-space PML4 entries. These are now constants
defined in kernel_memory.h instead.

Tags: boot memory paging

src/kernel/page_table.h

@@ -15,7 +15,7 @@ struct page_table
 		return static_cast<level>(static_cast<unsigned>(l) + 1);
 	}
 
-	uint64_t entries[512];
+	uint64_t entries[memory::table_entries];
 
 	inline page_table * get(int i, uint16_t *flags = nullptr) const {
 		uint64_t entry = entries[i];