Merge branch 'bind_addr_zero'

Kuniyuki Iwashima says:

====================
Improve bind(addr, 0) behaviour.

Currently we fail to bind sockets to ephemeral ports when all of the ports
are exhausted even if all sockets have SO_REUSEADDR enabled. In this case,
we still have a chance to connect to the different remote hosts.

These patches add net.ipv4.ip_autobind_reuse option and fix the behaviour
to fully utilize all space of the local (addr, port) tuples.

Changes in v5:
  - Add more description to documents.
  - Fix sysctl option to use proc_dointvec_minmax.
  - Remove the Fixes: tag and squash two commits.

Changes in v4:
  - Add net.ipv4.ip_autobind_reuse option to not change the current behaviour.
  - Modify .gitignore for test.
  https://lore.kernel.org/netdev/20200308181615[email protected]/

Changes in v3:
  - Change the title and write more specific description of the 3rd patch.
  - Add a test in tools/testing/selftests/net/ as the 4th patch.
  https://lore.kernel.org/netdev/20200229113554[email protected]/

Changes in v2:
  - Change the description of the 2nd patch ('localhost' -> 'address').
  - Correct the description and the if statement of the 3rd patch.
  https://lore.kernel.org/netdev/20200226074631[email protected]/

v1 with tests:
  https://lore.kernel.org/netdev/20200220152020[email protected]/
====================

Reviewed-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>