Merge branch 'skb_frag_fill_page_desc'

Yunsheng Lin says:

====================
net: introduce skb_frag_fill_page_desc()

Most users use __skb_frag_set_page()/skb_frag_off_set()/
skb_frag_size_set() to fill the page desc for a skb frag.
It does not make much sense to calling __skb_frag_set_page()
without calling skb_frag_off_set(), as the offset may depend
on whether the page is head page or tail page, so add
skb_frag_fill_page_desc() to fill the page desc for a skb
frag.

In the future, we can make sure the page in the frag is
head page of compound page or a base page, if not, we
may warn about that and convert the tail page to head
page and update the offset accordingly, if we see a warning
about that, we also fix the caller to fill the head page
in the frag. when the fixing is done, we may remove the
warning and converting.

In this way, we can remove the compound_head() or use
page_ref_*() like the below case:
https://elixir.bootlin.com/linux/latest/source/net/core/page_pool.c#L881
https://elixir.bootlin.com/linux/latest/source/include/linux/skbuff.h#L3383



It may also convert net stack to use the folio easier.

V1: repost with all the ack/review tags included.
RFC: remove a local variable as pointed out by Simon.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>