Don't overflow the kernel struct mdio in the MDIOCLIST ioctl.

[brooks]

Summary

Always terminate the list with -1 and document the ioctl behavior.
This preserves existing behavior as seen from userspace with the
addition of the unconditional termination which will not be seen by
working consumers of MDIOCLIST.

Because this ioctl can only be performed by root (in default
configurations) and is not used in the base system this bug is not
deemed to warrant either a security advisory or an eratta notice.

[kib]

So the overflow can happen if the number of md's is greater than MONPAD, right ?

I would appreciate if the comment for md_pad in mdioctl.h was updated.

[brooks]

> MDNPAD - 1 (one slot lost to the count). I'll commit an update to md_pad's comment separately (I almost submitted it with this one, but it's only somewhat related).