scsi: libiscsi: use sendpage_ok() in iscsi_tcp_segment_map()

In iscsci driver, iscsi_tcp_segment_map() uses the following code to
check whether the page should or not be handled by sendpage:
    if (!recv && page_count(sg_page(sg)) >= 1 && !PageSlab(sg_page(sg)))

The "page_count(sg_page(sg)) >= 1 && !PageSlab(sg_page(sg)" part is to
make sure the page can be sent to network layer's zero copy path. This
part is exactly what sendpage_ok() does.

This patch uses  use sendpage_ok() in iscsi_tcp_segment_map() to replace
the original open coded checks.
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Lee Duncan <lduncan@suse.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Vasily Averin <vvs@virtuozzo.com>
Cc: Cong Wang <amwang@redhat.com>
Cc: Mike Christie <michaelc@cs.wisc.edu>
Cc: Chris Leech <cleech@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/scsi/libiscsi_tcp.c

@@ -128,7 +128,7 @@ static void iscsi_tcp_segment_map(struct iscsi_segment *segment, int recv)
 	 * coalescing neighboring slab objects into a single frag which
 	 * triggers one of hardened usercopy checks.
 	 */
-	if (!recv && page_count(sg_page(sg)) >= 1 && !PageSlab(sg_page(sg)))
+	if (!recv && sendpage_ok(sg_page(sg)))
 		return;
 
 	if (recv) {