Fix: reset out_fd_offset when we rotate the trace file

This value is only used for the sync_file_range, but it has to be set
to 0 when we start to write in a new trace file, otherwise the values
passed to this call are bogus.

Applied to 2.3, but could probably be backported to 2.2.

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