NetKGDB v3

[Andrei Warkentin <andrey.warkentin@gmail.com>]

Hi,

This is v3 of the tentative patch set I am sending for feedback.
It implements a network I/O backend for KDB/KGDB, the kernel
debugger.

NetKGDB is very similar to netconsole, on which it is based, but with
a crucial difference - netkgdb is designed to accept connections
from any hosts, i.e. it is not necessary to specify these ahead
of time. This makes it that much more useful in a "this host crashed
and a I want a developer to take a look at it" scenario, common to
large scale test/QA farms and automated testing harnesses. This
involves a minor change to the netpoll rx_hook, namely, to provide
source connection information.

This differs from v2 in adding the flush() callback in kgdb_io,
which would be necessary for correct behavior in gdbstub mode,
and enhances the debug core to support registering multiple
I/O ops. The later allows both kgdboc, netkgdb and any other
providers to coexist peacefully, and allow debugging both
interactively and via the network.

V2 cleaned up a lot of the strange cruft in v1 surrounding
dealing with netdev notifiers.

[PATCHv3 1/3] NETPOLL: Extend rx_hook support.
[PATCHv3 2/3] NETKGDB: Ethernet/UDP/IP KDB transport.
[PATCHv3 3/3] KGDB: Allow registering multiple I/O ops.

Thanks,
A