As diagnosed by Andreas, fs/btrfs/volumes.c:__btrfs_map_block()
calls
do_div(stripe_nr, stripe_len);
with stripe_len u64, while do_div() assumes the divisor is a 32-bit number.
Due to the lack of truncation in the m68k-specific implementation of
do_div(), the division is performed using the upper 32-bit word of
stripe_len, which is zero.
This was introduced by commit 53b381b3abeb86f12787a6c40fee9b2f71edc23b
("Btrfs: RAID5 and RAID6"), which changed the divisor from
map->stripe_len (struct map_lookup.stripe_len is int) to a 64-bit temporary.