igb: push data into first igb_tx_buffer sooner to reduce stack usage

Instead of storing most of the data for the TX hot path in the stack until
we are ready to write the descriptor we can save ourselves some time and
effort by pushing the SKB, tx_flags, gso_size, bytecount, and protocol into
the first igb_tx_buffer since that is where we will end up putting it
anyway.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>