bcachefs: Work around a zstd bug

bcachefs: Work around a zstd bug

The zstd compression code seems to have a bug where it will write just
past the end of the destination buffer - probably only when the
compressed output isn't going to fit in the destination buffer, which
will never happen if you're always allocating a bigger buffer than the
source buffer which would explain other users not hitting it. But, we
size the buffer according to how much contiguous space on disk we have,
so...

generally, bugs like this don't write more than a word past the end of
the buffer, so an easy workaround is to subtract a fudge factor from the
buffer size.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>