[NET]: Fix ___pskb_trim when entire frag_list needs dropping

When the trim point is within the head and there is no paged data,
___pskb_trim fails to drop the first element in the frag_list.
This patch fixes this by moving the len <= offset case out of the
page data loop.

This patch also adds a missing kfree_skb on the frag that we just
cloned.

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