net: add netmem to skb_frag_t

Use struct netmem* instead of page in skb_frag_t. Currently struct
netmem* is always a struct page underneath, but the abstraction
allows efforts to add support for skb frags not backed by pages.

There is unfortunately 1 instance where the skb_frag_t is assumed to be
a exactly a bio_vec in kcm. For this case, WARN_ON_ONCE and return error
before doing a cast.

Add skb[_frag]_fill_netmem_*() and skb_add_rx_frag_netmem() helpers so
that the API can be used to create netmem skbs.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>