ss: fix common node header size and check free space

ss: fix common node header size and check free space

COMMON_HEADER_SIZE had not been corrected after modifying strings
sections.
Check that node free space is consistent with what is written to
disk.

Change-Id: I9186f8b2e32159d9f2c771358c337aaf0ebb38c3
Signed-off-by: Loïc Prieur-Drevon <loic.prieurdrevon@gmail.com>
Reviewed-on: https://git.eclipse.org/r/83456
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI