[NET]: Add missing UFO initialisations

This bug was unknowingly fixed the GSO patches (or rather, its effect was
unknown at the time).

Thanks to Marco Berizzi's persistence which is documented in the thread
"ipsec tunnel asymmetrical mtu", we now know that it can have highly
non-obvious symptoms.

What happens is that uninitialised uso_size fields can cause packets to
be incorrectly identified as UFO, which means that it does not get
fragmented even if it's over the MTU.

The fix is simple enough.

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