bootloader: Round up heap region allocation sizes to B_PAGE_SIZE.

In testing, the allocation sizes I was seeing were all multiples
of the page size anyway, but of course there's no requirement
that they are, so we need to round here.

Pointed out by mmlr.

src/system/boot/arch/m68k/mmu.cpp

@@ -668,6 +668,7 @@ platform_free_region(void *address, size_t size)
 ssize_t
 platform_allocate_heap_region(size_t size, void **_base)
 {
+	size = ROUNDUP(size, B_PAGE_SIZE);
 	addr_t base = get_next_physical_address(size);
 	if (base == 0)
 		return B_NO_MEMORY;

src/system/boot/platform/amiga_m68k/mmu.cpp

@@ -666,6 +666,7 @@ platform_free_region(void *address, size_t size)
 ssize_t
 platform_allocate_heap_region(size_t size, void **_base)
 {
+	size = ROUNDUP(size, B_PAGE_SIZE);
 	addr_t base = get_next_physical_address(size);
 	if (base == 0)
 		return B_NO_MEMORY;

src/system/boot/platform/atari_m68k/mmu.cpp

@@ -671,6 +671,7 @@ platform_free_region(void *address, size_t size)
 ssize_t
 platform_allocate_heap_region(size_t size, void **_base)
 {
+	size = ROUNDUP(size, B_PAGE_SIZE);
 	addr_t base = get_next_physical_address(size);
 	if (base == 0)
 		return B_NO_MEMORY;

src/system/boot/platform/bios_ia32/mmu.cpp

@@ -826,6 +826,7 @@ platform_free_region(void *address, size_t size)
 ssize_t
 platform_allocate_heap_region(size_t size, void **_base)
 {
+	size = ROUNDUP(size, B_PAGE_SIZE);
 	addr_t base = allocate_physical(size, false);
 	if (base == 0)
 		return B_NO_MEMORY;