ppp: simplify input error handling

ppp: simplify input error handling

Currently, ppp_input_error() indicates an error by allocating a 0-length
skb and calling ppp_do_recv(). It takes an error code argument, which is
stored in skb->cb, but not used by ppp_receive_frame().

Simplify the error handling by removing the unused parameter and the
unnecessary skb allocation. Instead, call ppp_receive_error() directly
from ppp_input_error() under the recv lock, and the length check in
ppp_receive_frame() can be removed.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>