There were some overflows in the time conversion functions - fix this by
converting tv_sec and tv_nsec separately. Also, set sb->time_min and
sb->time_max.

Fixes xfstest generic/258.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>