Fix: Allow 64-bit packet offset

Offset within a packet is calculated in bits. Therefore, if, for an
unforeseen reason, a 32-bit system would happen to build babeltrace
without large file support, the maximum packet size would have been
limited to 256MB (4GB / 8 bits per byte), because we were using size_t.

Use 64-bit type to hold the offset within a packet from now on.

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