Implement nsecs_to_jiffies() in the LinuxKPI and while at it
streamline the rest of the xxx_to_jiffies() functions to have a
constant 64-bit argument and use identical range checks for the
result.

Specifically preserve msecs_to_jiffies(0) returning 0. See r282743 for
further details.

MFC after: 1 week
Sponsored by: Mellanox Technologies