atm: Revert atm_account_tx() if copy_from_iter_full() fails.

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

atm: Revert atm_account_tx() if copy_from_iter_full() fails.

In vcc_sendmsg(), we account skb->truesize to sk->sk_wmem_alloc by atm_account_tx().

It is expected to be reverted by atm_pop_raw() later called by vcc->dev->ops->send(vcc, skb).

However, vcc_sendmsg() misses the same revert when copy_from_iter_full() fails, and then we will leak a socket.

Let's factorise the revert part as atm_return_tx() and call it in the failure path.

Note that the corresponding sk_wmem_alloc operation can be found in alloc_tx() as of the blamed commit.

$ git blame -L:alloc_tx net/atm/common.c c55fa3cccbc2c~