Make sock_sendpage() use kernel_sendpage()

kernel_sendpage() does the proper default case handling for when the
socket doesn't have a native sendpage implementation.

Now, arguably this might be something that we could instead solve by
just specifying that all protocols should do it themselves at the
protocol level, but we really only care about the common protocols.
Does anybody really care about sendpage on something like Appletalk? Not
likely.
Acked-by: NDavid S. Miller <davem@davemloft.net>
Acked-by: NJulien TINNES <julien@cr0.org>
Acked-by: NTavis Ormandy <taviso@sdf.lonestar.org>
Cc: stable@kernel.org
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

net/socket.c

@@ -736,7 +736,7 @@ static ssize_t sock_sendpage(struct file *file, struct page *page,
 	if (more)
 		flags |= MSG_MORE;
 
-	return sock->ops->sendpage(sock, page, offset, size, flags);
+	return kernel_sendpage(sock, page, offset, size, flags);
 }
 
 static ssize_t sock_splice_read(struct file *file, loff_t *ppos,