Clear the page tables on creation, use macros for counts.

This makes it less likely that uninitialized entries cause troubles.
Also panic if we encounter an unknown entry type instead of defaulting
to 4K pages.

src/system/boot/arch/arm/arch_mmu.cpp

@@ -253,27 +253,38 @@ get_next_page_table(uint32 type)
 		sNextPageTableAddress, kPageTableRegionEnd, type));
 
 	size_t size = 0;
+	size_t entryCount = 0;
 	switch (type) {
 		case ARM_MMU_L1_TYPE_COARSE:
-		default:
 			size = ARM_MMU_L2_COARSE_TABLE_SIZE;
+			entryCount = ARM_MMU_L2_COARSE_ENTRY_COUNT;
 			break;
 		case ARM_MMU_L1_TYPE_FINE:
 			size = ARM_MMU_L2_FINE_TABLE_SIZE;
+			entryCount = ARM_MMU_L2_FINE_ENTRY_COUNT;
 			break;
 		case ARM_MMU_L1_TYPE_SECTION:
+			// TODO: Figure out parameters for section types.
 			size = 16384;
 			break;
+		default:
+			panic("asked for unknown page table type: %#" B_PRIx32 "\n", type);
+			return NULL;
 	}
 
 	addr_t address = sNextPageTableAddress;
-	if (address >= kPageTableRegionEnd) {
-		TRACE(("outside of pagetableregion!\n"));
-		return (uint32 *)get_next_physical_address_alligned(size, 0xffffffc0);
+	if (address < kPageTableRegionEnd)
+		sNextPageTableAddress += size;
+	else {
+		TRACE(("page table allocation outside of pagetable region!\n"));
+		address = get_next_physical_address_alligned(size, 0xffffffc0);
 	}
 
-	sNextPageTableAddress += size;
-	return (uint32 *)address;
+	uint32 *pageTable = (uint32 *)address;
+	for (size_t i = 0; i < entryCount; i++)
+		pageTable[i] = 0;
+
+	return pageTable;
 }