client: clamp I/O sizes to INT_MAX when we can't return larger values

client: clamp I/O sizes to INT_MAX when we can't return larger values

We have several API functions that allow the caller to request I/Os
larger than INT_MAX bytes, but that return an int. Ensure that we don't
try to do more I/O than we can represent in the return value.

Tracker: http://tracker.ceph.com/issues/22948
Signed-off-by: Jeff Layton <jlayton@redhat.com>