ceph: cleanup types in striped_read()

We pass in a u64 value for "len" and then immediately truncate away the
upper 32 bits.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <alex.elder@linaro.org>
possible btrfs fix from chris