net: pskb_expand_head() optimization

pskb_expand_head() blindly takes references on fragments before calling
skb_release_data(), potentially releasing these references.

We can add a fast path, avoiding these atomic operations, if we own the
last reference on skb->head.

Based on a previous patch from David

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>