rbd: Fix ceph_snap_context size calculation

(cherry picked from commit f9f9a1904467816452fc70740165030e84c2c659)

ceph_snap_context->snaps is an u64 array

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>