buffer to reallocate
room to add at head
room to add at tail
allocation priority
Expands (or creates identical copy, if &nhead and &ntail are zero) header of skb. &sk_buff itself is not changed. &sk_buff MUST have reference count of 1. Returns zero in the case of success or error, if expansion failed. In the last case, &sk_buff is not changed.
All the pointers pointing into skb header may change and must be reloaded after call to this function.