net/sched: netem: handle multi-segment skb in corruption

net/sched: netem: handle multi-segment skb in corruption

The packet corruption code only flipped bits in the linear
header portion of the skb, skipping corruption when
skb_headlen() was zero.

Linearize the whole skb if necessary before corruption.
Extends d64cb81dcbd5 ("net/sched: sch_netem: fix out-of-bounds access
in packet corruption") with a more general solution.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Link: https://patch.msgid.link/20260509171123.307549-5-stephen@networkplumber.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>