tcp: take care of mixed splice()/sendmsg(MSG_ZEROCOPY) case

In the Linux kernel, the following vulnerability has been resolved:

tcp: take care of mixed splice()/sendmsg(MSG_ZEROCOPY) case

syzbot found that mixing sendpage() and sendmsg(MSG_ZEROCOPY)

calls over the same TCP socket would again trigger the

infamous warning in inet_sock_destruct()

WARN_ON(sk_forward_alloc_get(sk));

While Talal took into account a mix of regular copied data

and MSG_ZEROCOPY one in the same skb, the sendpage() path

has been forgotten.

We want the charging to happen for sendpage(), because

pages could be coming from a pipe. What is missing is the

downgrading of pure zerocopy status to make sure

sk_forward_alloc will stay synced.

Add tcp_downgrade_zcopy_pure() helper so that we can

use it from the two callers.