Bees: fix [-Werror=implicit-fallthrough=]

In gcc 7+ warning: implicit-fallthrough has been added
In some places fallthrough is expectable, disable warning

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>

src/bees-roots.cc

@@ -768,6 +768,7 @@ BeesCrawl::fetch_extents()
 				break;
 			case BTRFS_FILE_EXTENT_PREALLOC:
 				BEESCOUNT(crawl_prealloc);
+				// fallthrough
 			case BTRFS_FILE_EXTENT_REG: {
 				auto physical = call_btrfs_get(btrfs_stack_file_extent_disk_bytenr, i.m_data);
 				auto ram = call_btrfs_get(btrfs_stack_file_extent_ram_bytes, i.m_data);