Merge branch 'review/next/phy-index-v4' into next

Maxime Chevallier:

This series adds the ethtool-side support to list PHYs associated to a
netdevice, as well as allowing to target PHYs for some commands :
 - PSE-PD commands
 - Cable testing commands
 - PLCA commands

The V4 addresses all of Michal's reviews this time, introducing better error
messages, documenting in the manpage the possible values for the 0 phy index,
and making sure ethtool can still build without libmnl.

The PHY-targetting commands look like this:

ethtool --phy 1 --cable-test eth0

Note that the --phy parameter gets passed at the beginning of the
command-line. This allows getting a generic command-line parsing code,
easy to write, but at the expense of maybe being a bit counter intuitive.

Another option could be to add a "phy" parameter to all the supported
commands, let me know if you think this looks too bad.

Patch 1 deals with the ability to pass a PHY index to the relevant
commands.

Patch 2 implements the --show-phys command. This command uses a netlink
DUMP request to list the PHYs, and introduces the ability to perform
filtered DUMP request, where the netdev index gets passed in the DUMP
request header.