client: type safety cleanup for _read and _write codepaths

client: type safety cleanup for _read and _write codepaths

In the read codepath, bl->length() returns an unsigned value, and that
could end up looking negative when cast to int. On the write side,
totalwritten is a uint64_t, which could look negative when cast to int.

Have the underlying layers pass back an int64_t and convert them to
int at a higher level. This prepares the underlying infrastructure for
ceph_ll_readv and ceph_ll_writev support.

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