[NET] netloop: Do not clobber cloned skb page frags

[NET] netloop: Do not clobber cloned skb page frags

The netloop driver tries to localise foreign mappings by
copying them.  Unfortunately, it does so by directly modifying
skb page frags without checking whether the skb is cloned or
not.  In fact, the packet is going to be cloned more often
than not.

This may result in either data corruption on DMA or a
page fault in dom0 which kills the whole machine.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>