MINOR: stats-file: reserve some bytes in exported structs

MINOR: stats-file: reserve some bytes in exported structs

We may need additional struct members in shm_stats_file_object and
shm_stats_file_hdr, yet since these structs are exported they should
not change in size nor ordering else it would require a version change
to break compability on purpose since mapping would differ.

Here we reserve 64 additional bytes in shm_stats_file_object, and
128 bytes in shm_stats_file_hdr for future usage.