netem: slotting with non-uniform distribution

Extend slotting with support for non-uniform distributions. This is
similar to netem's non-uniform distribution delay feature.

Commit f043efeae2f1 ("netem: support delivering packets in delayed
time slots") added the slotting feature to approximate the behaviors
of media with packet aggregation but only supported a uniform
distribution for delays between transmission attempts. Tests with TCP
BBR with emulated wifi links with non-uniform distributions produced
more useful results.

Syntax:
   slot dist DISTRIBUTION DELAY JITTER [packets MAX_PACKETS] \
      [bytes MAX_BYTES]

The syntax and use of the distribution table is the same as in the
non-uniform distribution delay feature. A file DISTRIBUTION must be
present in TC_LIB_DIR (e.g. /usr/lib/tc) containing numbers scaled by
NETEM_DIST_SCALE. A random value x is selected from the table and it
takes DELAY + ( x * JITTER ) as delay. Correlation between values is not
supported.

Examples:
  Normal distribution delay with mean = 800us and stdev = 100us.
  > tc qdisc add dev eth0 root netem slot dist normal 800us 100us

  Optionally set the max slot size in bytes and/or packets.
  > tc qdisc add dev eth0 root netem slot dist normal 800us 100us \
    bytes 64k packets 42

Signed-off-by: Yousuk Seung <ysseung@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>