Some systems use a native 64 KiB page size, which means that the bitmap for the smallest size class can be 8192 bits, not just 512 bits as when the page size is 4 KiB. Linear search in bitmap_{sfu,ffu}() is unacceptably slow for such large bitmaps. This reverts commit 7c00f04ff40a34627e31488d02ff1081c749c7ba.