libceph: use sendpage_ok() in ceph_tcp_sendpage()

In libceph, ceph_tcp_sendpage() does the following checks before handle
the page by network layer's zero copy sendpage method,
	if (page_count(page) >= 1 && !PageSlab(page))

This check is exactly what sendpage_ok() does. This patch replace the
open coded checks by sendpage_ok() as a code cleanup.
Signed-off-by: Coly Li <colyli@suse.de>
Acked-by: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ceph/messenger.c

@@ -575,7 +575,7 @@ static int ceph_tcp_sendpage(struct socket *sock, struct page *page,
 	 * coalescing neighboring slab objects into a single frag which
 	 * triggers one of hardened usercopy checks.
 	 */
-	if (page_count(page) >= 1 && !PageSlab(page))
+	if (sendpage_ok(page))
 		sendpage = sock->ops->sendpage;
 	else
 		sendpage = sock_no_sendpage;