bcachefs: Add a better limit for maximum number of buckets

The bucket_gens array is a single array allocation (one byte per
bucket), and kernel allocations are still limited to INT_MAX.

Check this limit to avoid failing the bucket_gens array allocation.

Reported-by:  <syzbot+b29f436493184ea42e2b@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>