igb: Save more Tx states

igb: Save more Tx states

The current implementation of igb uses only part of a advanced Tx
context descriptor and first data descriptor because it misses some
features and sniffs the trait of the packet instead of respecting the
packet type specified in the descriptor. However, we will certainly
need the entire Tx context descriptor when we update igb to respect
these ignored fields. Save the entire context descriptor and first
data descriptor except the buffer address to prepare for such a change.

This also introduces the distinction of contexts with different
indexes, which was not present in e1000e but in igb.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Signed-off-by: Jason Wang <jasowang@redhat.com>