drbd: code cleanup by using sendpage_ok() to check page for kernel_sendpage()

In _drbd_send_page() a page is checked by following code before sending
it by kernel_sendpage(),
(page_count(page) < 1) || PageSlab(page)
If the check is true, this page won't be send by kernel_sendpage() and
handled by sock_no_sendpage().

This kind of check is exactly what macro sendpage_ok() does, which is
introduced into include/linux/net.h to solve a similar send page issue
in nvme-tcp code.

This patch uses macro sendpage_ok() to replace the open coded checks to
page type and refcount in _drbd_send_page(), as a code cleanup.

Signed-off-by: Coly Li <[e-mail]>
Cc: Philipp Reisner <[e-mail]>
Cc: Sagi Grimberg <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/block/drbd/drbd_main.c

···
 	 * put_page(); and would cause either a VM_BUG directly, or
 	 * __page_cache_release a page that would actually still be referenced
 	 * by someone, leading to some obscure delayed Oops somewhere else. */
-	if (drbd_disable_sendpage || (page_count(page) < 1) || PageSlab(page))
+	if (drbd_disable_sendpage || !sendpage_ok(page))
 		return _drbd_no_send_page(peer_device, page, offset, size, msg_flags);
 
 	msg_flags |= MSG_NOSIGNAL;