The function makes a sense only on a fragmented &sk_buff, it expands header moving its tail forward and copying necessary data from fragmented part.
&sk_buff MUST have reference count of 1.
Returns NULL (and &sk_buff does not change) if pull failed
or value of new tail of skb in the case of success.
All the pointers pointing into skb header may change and must be reloaded after call to this function.