tc: Correct convert ticks to msecs on parsing tc TM

tc: Correct convert ticks to msecs on parsing tc TM

Use sysconf(_SC_CLK_TCK) to read run time "number of clock ticks per
second" and use that to convert ticks to msecs.
This is how iproute does the conversion when parsing tc filters.
The system call is done only once.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>