selftests: Move two functions from mlxsw's qos_lib to lib

The function humanize() is used for converting value in bits/s to a
human-friendly approximate value in Kbps, Mbps or Gbps. There is nothing
hardware-specific in that, so move the function to lib.sh.

Similarly for the rate() function, which just does a bit of math to
calculate a rate, given two counter values and a time interval.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>