The littlefs allows buffers to be passed statically in the case
that a system does not have a heap. Unfortunately, this means we
can't round up in the case of an unaligned lookahead buffer.
Double unfortunately, rounding down after clamping to the block device
size could result in a lookahead of zero for block devices < 32 blocks
large.
The assert in littlefs does catch this case, but rounding down prevents
support for < 32 block devices.
The solution is to simply require a 32-bit aligned buffer with an
assert. This avoids runtime problems while allowing a user to pass
in the correct buffer for < 32 block devices. Rounding up can be
handled at higher API levels.
Simply limiting the lookahead region to the size of
the block device fixes the problem.
Also added logic to limit the allocated region and
floor to nearest word, since the additional memory
couldn't really be used effectively.
When the lookahead buffer wraps around in an unaligned filesystem, it's
possible for blocks at the beginning of the disk to have a negative distance
from the lookahead, but still reside in the lookahead buffer.
Switching to signed modulo doesn't quite work due to how negative modulo
is implemented in C, so the simple solution is to shift the region to be
positive.
