rbd discard return -EINVAL if len > MAX_INT32

rbd discard return -EINVAL if len > MAX_INT32

rbd discard use 'int' to return discarded length, but the 'len' user
passed is 'uint64', in some case, the ret value will be truncated
and return a negative value which means discard failed. ret -EINVAL
if len > MAX_INT32 to indicate support len <= MAX_INT32 only.

Fixes: http://tracker.ceph.com/issues/16465
Signed-off-by: Huan Zhang <zhanghuan@chinac.com>