fattime: fix fattime to timespec conversion of dates beyond 2106-02-06

It turns out that the only conversion issue was in fattime2timespec, where
multiplying the number of seconds in a day by the number of days overflowed
32-bit unsigned int for dates beyond 2106-02-07 06:28:15.

Casting one of the multiplicands as time_t forces a 64-bit multiplication on
systems where time_t is 64-bits and produces no binary changes on the one
remaining system with 32-bit time_t (namely i386).

Since the code is now tested & fixed, this change removes the fixme comments.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44755