MEDIUM: stats-file: add some BUG_ON() guards to ensure exported structs are not chang...

MEDIUM: stats-file: add some BUG_ON() guards to ensure exported structs are not changed by accident

Add two BUG_ON() in shm_stats_file_prepare() which will trigger if
exported structures (shm_stats_file_hdr and shm_stats_file_object) change
in size, because it means that they will become incompatible with older
versions and thus precautions should be taken by the developer to ensure
compatibility with olders versions, or at least detect incompatible
versions by changing the version number to prevent bugs resulting
from inconsistent mapping between versions. The BUG_ON() may be
safely adjusted then.

Please note that it doesn't protect against accidental struct member
re-ordering if the resulting struct size is equal..