rbd_fsx: do not exceed 250GB for fsx image

This breaks on 32-bit architectures because fsx can't allocate the
in-memory buffer or hit some other limit.  500MG also seems to run out of
memory (tho it at least does not segfault).

Fixes: #6576

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #143 from ceph/wip-6576

fix rbd fsx on 32-bit

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>