Fix data_offset when importing the indexes

Fix data_offset when importing the indexes

When importing an index, we don't have the data_offset and we want to
mmap the tracefile only when needed, so we introduce a new function to
only compute this information when we are going to use a new packet.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>