Negative tv_sec bypasses 1980-truncate guard in timespec2fattime

Summary

timespec2fattime(:157) t2=t1/DAY. Negative t1 -> negative/unsigned wraps huge -> t2<T1980 false -> 1980-guard bypassed. dtp/dhp(:150-154) no pre-epoch guard at all. Garbage FAT timestamp fields. Kernel timestamps always>=epoch, very low reachability.