net: minor cleanup in skb_ext_add()

When the extension to be added is already present, the only
skb field we may need to update is 'extensions': we can reorder
the code and avoid a branch.

v1 -> v2:
 - be sure to flag the newly added extension as active

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>