svcrdma: Combine list fields in struct svc_rdma_op_ctxt

Clean up: The free list and the dto_q list fields are never used at
the same time. Reduce the size of struct svc_rdma_op_ctxt by
combining these fields.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>