net: use %pS for kfree_skb tracing event location

For the cases where 'reason' doesn't give any clue, it's still
nice to be able to track the kfree_skb caller location. %p doesn't
help much so let's use %pS which prints the symbol+offset.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20221123040947.1015721-1-sdf@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>