Ceph » Linux kernel client

I added code to in the client messenger to verify auth_client->ops
and auth_client->ops->(method) were non-null before blindly
dereferencing these pointers. In review, Sage suggested that some
of these could be BUG_ON() instead.

It would be preferable to further constrain things so that is possible,
but at the moment I don't have complete enough knowledge to make that
switch with confidence it's correct.

So at some point, I'd like to do this. And if in certain cases BUG_ON()
is inappropriate then other validity checks might be used.