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

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~

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Simon Horman <horms@kernel.org>
Closes: https://lore.kernel.org/netdev/20250614161959.GR414686@horms.kernel.org/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250616182147.963333-3-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>