pppoe: Fix memory leak in pppoe_sendmsg()

pppoe: Fix memory leak in pppoe_sendmsg()

syzbot reports a memory leak in pppoe_sendmsg [1].

The problem is in the pppoe_recvmsg() function that handles errors
in the wrong order. For the skb_recv_datagram() function, check
the pointer to skb for NULL first, and then check the 'error' variable,
because the skb_recv_datagram() function can set 'error'
to -EAGAIN in a loop but return a correct pointer to socket buffer
after a number of attempts, though 'error' remains set to -EAGAIN.

skb_recv_datagram
      __skb_recv_datagram          // Loop. if (err == -EAGAIN) then
                                   // go to the next loop iteration
          __skb_try_recv_datagram  // if (skb != NULL) then return 'skb'
                                   // else if a signal is received then
                                   // return -EAGAIN

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with Syzkaller.

Link: https://syzkaller.appspot.com/bug?extid=6bdfd184eac7709e5cc9
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+6bdfd184eac7709e5cc9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6bdfd184eac7709e5cc9
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20240214085814.3894917-1-Ilia.Gavrilov@infotecs.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>