ctf: read packet sequence number from index

Starting with version 1.1, the CTF index format includes a packet
sequence number.  Read it, if the index minor version is >=1.  The index
major version is already checked to always be 1 above in the code.

If the version is lower than that, store UINT64_MAX in the
packet_seq_num field of the ctf_fs_ds_index_entry structure.

This information will be used in a subsequent patch.

Change-Id: I7b94a94344b26638ae4b7cfc4659067bebc7e195
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>