Revert "net: Reset secmark when scrubbing packet"

This patch reverts commit b8fb4e06
because the secmark must be preserved even when a packet crosses
namespace boundaries.  The reason is that security labels apply to
the system as a whole and is not per-namespace.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>