xprtrdma: Refactor __fmr_dma_unmap()

Separate the DMA unmap operation from freeing the MW. In a
subsequent patch they will not always be done at the same time,
and they are not related operations (except by order; freeing
the MW must be the last step during invalidation).
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/fmr_ops.c

@@ -186,15 +186,10 @@ static void
 __fmr_dma_unmap(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr_seg *seg)
 {
 	struct ib_device *device = r_xprt->rx_ia.ri_device;
-	struct rpcrdma_mw *mw = seg->rl_mw;
 	int nsegs = seg->mr_nsegs;
 
-	seg->rl_mw = NULL;
-
 	while (nsegs--)
 		rpcrdma_unmap_one(device, seg++);
-
-	rpcrdma_put_mw(r_xprt, mw);
 }
 
 /* Invalidate all memory regions that were registered for "req".
@@ -237,9 +232,11 @@ fmr_op_unmap_sync(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req)
 		seg = &req->rl_segments[i];
 
 		__fmr_dma_unmap(r_xprt, seg);
+		rpcrdma_put_mw(r_xprt, seg->rl_mw);
 
 		i += seg->mr_nsegs;
 		seg->mr_nsegs = 0;
+		seg->rl_mw = NULL;
 	}
 
 	req->rl_nchunks = 0;