test_librbd_fsx: fix sign-compare gcc warning

{read,write}bdy are signed, sizeof returns unsigned, gcc complains.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1956 from ceph/wip-fsx-sizeof

test_librbd_fsx: fix sign-compare gcc warning

Reviewed-by: Sage Weil <sage@inktank.com>