nvme-tcp: don't use sendpage for SLAB pages

According to commit a10674b ("tcp: detecting the misuse of
.sendpage for Slab objects") and previous discussion, tcp_sendpage
should not be used for pages that is managed by SLAB, as SLAB is not
taking page reference counters into consideration.

Signed-off-by: Mikhail Skorzhinskii <mskorzhinskiy@solarflare.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/tcp.c

@@ -860,7 +860,14 @@ static int nvme_tcp_try_send_data(struct nvme_tcp_request *req)
 		else
 			flags |= MSG_MORE;
 
-		ret = kernel_sendpage(queue->sock, page, offset, len, flags);
+		/* can't zcopy slab pages */
+		if (unlikely(PageSlab(page))) {
+			ret = sock_no_sendpage(queue->sock, page, offset, len,
+					flags);
+		} else {
+			ret = kernel_sendpage(queue->sock, page, offset, len,
+					flags);
+		}
 		if (ret <= 0)
 			return ret;