Fix: race with index file creation

Only the data thread should create the index file since it is
responsible of filling the FD field of the index struct in memory.
When the control thread is ready to write the index, the FD is already
set.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>