[PATCH for-4.7] nvme: Remove RCU namespace protection

[Sagi Grimberg]

> We can't sleep with RCU read lock held, but we need to do potentially
> blocking stuff to namespaces while traversing the list.
>
> This patch removes the rcu read locking to make this work, and leverages
> how namespace list manipulation currently occurs in order to be safe. Only
> scan work, reset work, or device removal can manipulate the namespace
> list, and none of those can execute at the same time. So, this patch
> locks the namespace list mutex only when the list is being changed,
> or when iterating the list by a non-IO task, like controller reset. We
> can't hold the lock for IO because we won't be able to clean up IO
> if it fails, so all those paths rely on the state machine to prevent
> two tasks from corrupting the list.
>
> This fixes these two BUGs:

This looks much better Keith,

Reviewed-by: Sagi Grimberg <sagi at grimberg.me>