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>