Extend the range of the return value from nsecs_to_jiffies64() to support
Mesa's drm_syncobj usage, in the LinuxKPI.

While at it optimise the jiffies conversion functions to avoid repeated
and constant calculations.

Submitted by: Greg V <greg@unrelenting.technology>
Differential Revision: https://reviews.freebsd.org/D23846
MFC after: 1 week
Sponsored by: Mellanox Technologies