svcrdma: Clean up svc_rdma_build_read_chunk()

Dan Carpenter <dan.carpenter@oracle.com> observed that the while()
loop in svc_rdma_build_read_chunk() does not document the assumption
that the loop interior is always executed at least once.

Defensive: the function now returns -EINVAL if this assumption
fails.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>