ceph: fix rsize/wsize capping in ceph_direct_read_write()
rsize/wsize cap should be applied before ceph_osdc_new_request() is
called. Otherwise, if the size is limited by the cap instead of the
stripe unit, ceph_osdc_new_request() would setup an extent op that is
bigger than what dio_get_pages_alloc() would pin and add to the page
vector, triggering asserts in the messenger.