net: decouple skb_frag_t from struct bio_vec

net: decouple skb_frag_t from struct bio_vec

There are some hidden dependencies between the size of skb_frag_t, and
hence also struct bio_vec, which causes network data corruption when
the bio_vec is changed. This is at least true on igb, unsure how many
drivers it affects or if it's a core feature that igb just happens to
use.

Since nothing ever came of the skb_frag_t and bio_vec unification, just
add a private skb_frag_struct instead that has the same layout as the
bio_vec.

Signed-off-by: Jens Axboe <axboe@kernel.dk>