idpf: simplify and fix splitq Tx packet rollback error path

Move (and rename) the existing rollback logic to singleq.c since that
will be the only consumer. Create a simplified splitq specific rollback
function to loop through and unmap tx_bufs based on the completion tag.
This is critical before replacing the Tx buffer ring with the buffer
pool since the previous rollback indexing will not work to unmap the
chained buffers from the pool.

Cache the next_to_use index before any portion of the packet is put on
the descriptor ring. In case of an error, the rollback will bump tail to
the correct next_to_use value. Because the splitq path now supports
different types of context descriptors (and potentially multiple in the
future), this will take care of rolling back any and all context
descriptors encoded on the ring for the erroneous packet. The previous
rollback logic was broken for PTP packets since it would not account for
the PTP context descriptor.

Fixes: 1a49cf81 ("idpf: add Tx timestamp flows")
Signed-off-by: Joshua Hay <joshua.a.hay@intel.com>
Reviewed-by: Madhu Chittim <madhu.chittim@intel.com>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>