bees: adjust BLOCK_SIZE_MAX_EXTENT to work around kernel bug

bees: adjust BLOCK_SIZE_MAX_EXTENT to work around kernel bug

In kernel commit 24542bf7ea5e4fdfdb5157ff544c093fa4dcb536 ("btrfs:
limit fallocate extent reservation to 256MB"), the maximum size of a
prealloc extent is set to 256MB.  This exceeds BTRFS_MAX_EXTENT_SIZE,
which is 128MB.

This causes some problems with parts of bees code which guard against
problems that arise when metadata is corrupted.  One manifestation of
that problem is a potential infinite (or very large finite) loop, caused
by the extent mapping code repeatedly trying to compute a dedupe solution
for a >128M extent, but being unable to do so because it is interrupted
when it hits the self-imposed 128M extent size limit.  This may be the
cause of issues such as [#325](https://github.com/Zygo/bees/issues/325).

Since the kernel bug is 14 years old, it's de facto part of the
filesystem spec now.  Increase the limit to the largest expected size
of btrfs extents.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>