xprtrdma: Fix a maybe-uninitialized compiler warning

This minor fix-up keeps GCC from complaining that "last' may be used
uninitialized", which breaks some build workflows that have been running
with all warnings treated as errors.
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xprtrdma/frwr_ops.c

@@ -515,8 +515,8 @@ void frwr_unmap_sync(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req)
 	 * a single ib_post_send() call.
 	 */
 	prev = &first;
-	while ((mr = rpcrdma_mr_pop(&req->rl_registered))) {
-
+	mr = rpcrdma_mr_pop(&req->rl_registered);
+	do {
 		trace_xprtrdma_mr_localinv(mr);
 		r_xprt->rx_stats.local_inv_needed++;
 
@@ -533,7 +533,8 @@ void frwr_unmap_sync(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req)
 
 		*prev = last;
 		prev = &last->next;
-	}
+	} while ((mr = rpcrdma_mr_pop(&req->rl_registered)));
+
 	mr = container_of(last, struct rpcrdma_mr, mr_invwr);
 
 	/* Strong send queue ordering guarantees that when the
@@ -617,8 +618,8 @@ void frwr_unmap_async(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req)
 	 * a single ib_post_send() call.
 	 */
 	prev = &first;
-	while ((mr = rpcrdma_mr_pop(&req->rl_registered))) {
-
+	mr = rpcrdma_mr_pop(&req->rl_registered);
+	do {
 		trace_xprtrdma_mr_localinv(mr);
 		r_xprt->rx_stats.local_inv_needed++;
 
@@ -635,7 +636,7 @@ void frwr_unmap_async(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req)
 
 		*prev = last;
 		prev = &last->next;
-	}
+	} while ((mr = rpcrdma_mr_pop(&req->rl_registered)));
 
 	/* Strong send queue ordering guarantees that when the
 	 * last WR in the chain completes, all WRs in the chain