include/linux/loop.h in busybox is obsolete (does not contain definitions of
struct loop_info64, LOOP_SET_STATUS64, LOOP_GET_STATUS64) and even wrong:

busybox copy:

/* Loop flags */
#define LO_FLAGS_DO_BMAP        1
#define LO_FLAGS_READ_ONLY      2
#define LO_FLAGS_BH_REMAP       4

current kernel:

/*
 * Loop flags
 */
enum {
        LO_FLAGS_READ_ONLY      = 1,
        LO_FLAGS_USE_AOPS       = 2,
};

This file breaks [mkinitrd-]busybox build with dietlibc.