[PATCH 0/6 v2][BFGH][linux] Fix kdump failures

[Tim Gardner]

v2 - proper subject lines for the various patches.

BugLink: https://bugs.launchpad.net/bugs/1927518

[Impact]

Microsoft has reported that a customer is not able to acquire a kdump
when a guest crashes.

SF: #00310145
https://canonical.lightning.force.com/lightning/r/Case/5004K000005pZNNQA2

The following 3 patches fix the issue. Patch #1 solves a problem where the
“Unable to send packet via vmbus” message is output continuously. But with
that problem fixed, the second problem can occur where the kdump kernel
panics due to receiving an unexpected VMbus UNLOAD complete message.

Patch #2 prevents the UNLOAD complete message from ever occurring in the
kdump kernel. But if the UNLOAD complete message does occur at some
unexpected time, Patch #3 prevents it from causing a panic.

[Test Plan]

Cause a guest kernel crash and acquire a kdump.

[Where problems could occur]

The increased Hyper-V timeout could have side effects.

[Other Info]

If these patches are deemed unsuitable for master, then I would at least like to have them
applied to fgh:linux-azure and bionic:linux-azure-4.15

The Bionic patches are also all clean cherry-picks, but 'git am' would not apply
the third of the FGH patches. There is likely some option to 'git am' that I'm
too lazy to find or remember.